6 Best Auxiliary RV Fuel Tanks For Diesel Engines That Extend Your Range

You’re in the middle of nowhere, west Texas or eastern Montana, and the fuel light blinks on. The next diesel station is 70 miles away, and you’re towing a 15,000-pound fifth wheel up a steady grade. This is the kind of anxiety that can take all the joy out of an RV adventure. An auxiliary fuel tank is your ticket out of that scenario, turning range anxiety into range confidence.

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Why You Need an Auxiliary Diesel Tank for Your RV

The most obvious reason is a massive increase in your driving range. Instead of a 400-mile range, you could suddenly have an 800 or even 1,200-mile range. This means you can bypass the often overpriced and inconvenient fuel stations right off the interstate and plan your stops on your own terms.

This freedom is about more than just convenience. It unlocks a different style of travel. You can venture deeper into national forests or explore remote BLM land without constantly calculating if you have enough fuel to get back out. It also lets you be a strategic buyer, filling up both tanks where diesel is cheapest, saving you hundreds of dollars over a long trip.

But let’s be clear: this isn’t for everyone. If you primarily stick to RV parks and well-traveled corridors, the cost and weight might not be worth it. An auxiliary tank is for the serious RVer—the boondocker, the full-timer, the cross-country traveler who sees the open road as a challenge to be met with preparation and peace of mind.

Transfer Flow Trax 4: Smart Fuel Management

Transfer Flow is a premium name in the auxiliary tank world, and for good reason. They aren’t just selling a steel box; they’re selling a fully integrated fuel management system. Their tanks are known for being incredibly well-built, often from 14-gauge aluminized steel for corrosion resistance.

The real magic is their TRAX 4 computer-controlled system. This isn’t a simple gravity feed or a manual switch. The system automatically monitors your main tank’s fuel level and pumps fuel from the auxiliary tank to keep the main tank full. You never have to think about it. The in-cab LCD screen shows you fuel levels in both tanks, giving you a complete picture of your range at a glance.

Of course, this level of technology and build quality comes at a price. Transfer Flow tanks are among the most expensive on the market. The added electronics also mean another potential point of failure, though their systems are known for reliability. For the RVer who wants a seamless, "set it and forget it" solution and is willing to invest in the best, Transfer Flow is often the top choice.

Titan Fuel Tanks: Durable Polymer Construction

While most tanks are made of metal, Titan takes a different approach. Their tanks are made from a military-grade, cross-linked polyethylene (polymer). Don’t let the word "plastic" fool you; this material is incredibly tough, capable of withstanding extreme impacts and temperatures without cracking.

The primary advantage of this polymer construction is that it’s completely corrosion-proof. There’s no rust to worry about, inside or out, and no interior lining to delaminate and clog your fuel filters. They are also often lighter than their steel counterparts, which is always a bonus when you’re watching your RV’s weight limits.

Titan offers two main solutions for RVers. They have traditional in-bed auxiliary tanks, but they are also famous for their extra-large, direct-replacement tanks. This option is fantastic for someone who wants to maintain a clean, empty truck bed. You simply swap out your stock 30-gallon tank for a 60-gallon Titan tank that fits in the exact same spot. The choice comes down to whether you want to add capacity or replace and upgrade it.

RDS Aluminum Combo Tank: Fuel and Tool Storage

For many RVers, especially those in a tow vehicle, bed space is precious. This is where a combo tank from RDS shines. They cleverly integrate a toolbox and an auxiliary fuel tank into a single, efficient unit. This means you don’t have to choose between carrying extra fuel and having a secure place for your tools, chocks, and recovery gear.

These units are typically built from heavy-duty aluminum with a diamond plate finish, making them both durable and rust-resistant. The fuel tank sits in the lower portion of the unit, while a lockable toolbox sits on top. It’s a simple, robust design that solves two problems at once, making it a favorite for full-timers and working RVers.

The tradeoff is a slight reduction in fuel capacity for a given footprint. A dedicated 50-gallon fuel tank will take up less space than a 50-gallon combo unit. However, the utility of having integrated, weatherproof storage is often worth sacrificing a few gallons of fuel. It’s a practical solution for maximizing the utility of your truck bed.

ATTA Fuel Shotz: Compact In-Bed Tank Solution

Not everyone needs a massive 100-gallon tank that dominates their truck bed. Sometimes, you just need an extra 30 or 40 gallons to bridge the gap between remote fuel stops. This is the niche that ATTA Fuel Shotz fills perfectly. Their tanks are designed to be compact and low-profile.

The key feature of many ATTA tanks is their shape and size, which often allows them to fit underneath a tonneau cover. This is a huge benefit for those who want to keep their truck bed secure and out of the elements. It provides the extra range you need without broadcasting to the world that you have an expensive auxiliary fuel system.

This is the ideal solution for the RVer who also uses their truck as a daily driver. You get the extended range for your towing trips, but you retain most of the utility and security of your truck bed for everyday tasks. It’s a compromise, but for many, it’s the most practical one.

Dee Zee Combo Tank: A Versatile, Trusted Option

Dee Zee is one of the most recognized names in truck accessories, and their auxiliary tanks live up to the brand’s reputation for quality and accessibility. They offer a range of combo tanks that provide a great balance of features, durability, and price, making them a solid, mainstream choice.

Like other combo tanks, Dee Zee units combine a fuel tank with a toolbox, typically constructed from Brite-Tread aluminum. They pay attention to the details, with features like lockable stainless steel paddle handles, internal baffles to reduce fuel sloshing, and a sturdy, welded construction. They are workhorses designed for real-world use.

Choosing a Dee Zee tank is often a decision based on trust and availability. You can find them at many automotive retailers, and their reputation means you know you’re getting a reliable product. While they may not have the high-tech features of a Transfer Flow or the custom options of KSH, they deliver excellent value and dependable performance.

KSH Auxiliary Tank: Custom-Fit Steel Designs

Sometimes, an off-the-shelf tank just won’t cut it. You might have a flatbed truck, a custom service body, or a unique set of dimensions you need to work around. This is where KSH comes in. They specialize in building custom, heavy-duty steel auxiliary tanks tailored to your specific needs.

KSH tanks are built from heavy-gauge steel and are fully baffled to control fuel movement. Their strength is their ability to build a tank to nearly any dimension or shape you require. Need a long, narrow tank to fit along the side of your flatbed? They can do it. Need a specific height to fit under a custom rack? They can do that, too.

The primary tradeoff is weight; steel is significantly heavier than aluminum or polymer. You also have to be mindful of the powder coat finish, as any deep scratches could lead to rust over time. But for the RVer with a non-standard setup or a need for ultimate durability, the customizability of a KSH tank is simply unmatched.

Installation & Legalities for Auxiliary Tanks

Let’s be very clear: installing an auxiliary fuel tank is not a simple weekend project. You are tapping into your vehicle’s fuel system. A mistake can lead to dangerous leaks, engine damage, or warranty voidance. Professional installation is strongly recommended.

There are two primary ways to get fuel from the auxiliary tank to your engine: an electric pump system or a gravity feed system. Electric pumps are safer and more controlled, while gravity feed kits are simpler. However, gravity feed systems are illegal for many modern, emissions-controlled diesel trucks and are not permitted by DOT regulations in many contexts because they can interfere with the vehicle’s evaporative emissions system. Always opt for a DOT-compliant installation kit with an electric pump and rollover safety valve.

Finally, the legalities are crucial. The tank itself must be DOT-approved for holding and transferring fuel. Regulations on fuel systems can vary by state, so it’s essential to check your local laws. A properly installed, high-quality system is an incredible asset. A poorly or illegally installed one is a serious liability.

Choosing an auxiliary tank is about matching the hardware to your travel style. It’s an investment not just in fuel capacity, but in freedom and security on the road. By understanding the tradeoffs between materials, features, and cost, you can find the right solution to stop worrying about the fuel gauge and start focusing on the adventure ahead.