Grey Water Tanks for Vans Explained
Fresh vs Gray Use and Why Most Vans Use a Combination of Both
Undermount water tanks are often discussed as an alternative to interior tanks, but in real world camper van builds they are more commonly used in combination with interior wheel well or center tanks.
Both tank types have advantages. The best systems do not choose one or the other. They intentionally use each where it makes the most sense.
This guide explains how undermount tanks are used for fresh and gray water, how they pair with interior tanks, and why hybrid systems are so common.
What Is a Van Undermount Water Tank?
An undermount water tank mounts underneath the van using rigid brackets tied into factory structure of the campervan.
Because it sits outside the insulated living space, it has fewer interior layout constraints but more exposure to weather, road debris, and freezing temperatures. NW Conversions manufactures water tanks to be thick to work efficient in these environments.
Why Most Vans Use Both Interior and Undermount Tanks
Most well designed camper vans use both an interior tank and an undermount tank. Interior tanks are typically used for fresh water because the water sits higher and closer to the pressure or gravity fill and naturally water falls down. Undermount tanks are commonly used for gray water or as supplemental fresh capacity.
Using both allows builders to balance space, capacity, and reliability.
Wheel Well and Center Tanks
Strong for Fresh Water but Layout Dependent
Wheel well and center mounted interior tanks work extremely well for fresh water. They stay warm, keep plumbing inside, and reduce freezing risk. The tradeoff is that they must fit within the cabinetry and layout of the van.
A common example is a 33 gallon right fresh water tank, which provides excellent capacity but requires intentional cabinet planning.
A slightly smaller 20 gallon center tank is often chosen when layouts are tighter or storage needs are higher.
Undermount Tanks
Undermount tanks offer more flexibility because they do not compete with interior cabinetry. For example, in a Transit, a 20 gallon driver side undermount tank is a common gray water choice that frees up interior space and allows gravity drainage. A 24 gallon spare tire water tank is often used as additional capacity, either for gray water or fresh water in warm weather builds.
Spare Tire Tanks and Tire Carrier Considerations
Spare tire water tanks use the space where the factory spare tire normally sits. Because of this, many vans using a spare tire tank. Note this also requires a rear tire ladder carrier or swing out carrier to relocate the spare. A good option for these are Rover Vans, Sandy Vans, or Owl.
This is not a drawback, but it is a planning consideration that should be made early in the build for budget purposes.
Fresh vs Gray Use in Hybrid Systems
In hybrid setups, roles are usually clear.
Interior wheel well or center tank
-Fresh water
Undermount tank
-Gray water or additional fresh
For example:
A 24 gallon center fresh tank paired with a 28 gallon spare tire tank offers large total capacity for longer trips.
A 20 gallon left or right fresh tank paired with a 22 gallon driver side undermount balances interior space with practical gray water management.
Both are valid approaches depending on how the van is used.
Pumps and Accumulators in Mixed Systems
Any fresh water tank located below fixtures requires a pump and accumulator. This is always true for undermount fresh tanks and often true for interior tanks as well. However, exterior tanks usually require more vertical lift and longer plumbing runs, which makes pump and accumulator selection more important.
In mixed systems, pumps are typically fed from the interior fresh tank, with exterior tanks acting as gray storage or supplemental supply.
Pressure Fill vs Gravity Fill Revisited
Pressure fill systems are preferred in hybrid setups. They allow water to be pushed cleanly into either tank without splash back and integrate well with pumps and accumulators.
Gravity fill can work, but it is more sensitive to slope and less predictable with exterior tanks.
Winter Considerations for Combination Systems
Interior fresh tanks are protected by cabin heat. Exterior tanks can be drained seasonally, used only for gray water, or heated with 12V tank heating pads when winter use is required.
This flexibility is one of the biggest advantages of a combined system.
Builder Perspective on Tank Combinations
From a builder standpoint, combination systems are the norm, not the exception. They allow layouts to stay clean, water to stay protected, and capacity to scale without overloading one part of the van.
The key is choosing roles intentionally rather than treating all tanks the same.
Final Thoughts on Undermount Tanks in Real Builds
Undermount water tanks are not an all or nothing decision. They work best when paired with interior tanks, each doing what it does best. Interior tanks provide protection and simplicity. Undermount tanks provide capacity and layout flexibility.
When combined thoughtfully, they create a water system that is easier to live with in every season.
