Promaster Undermount Grey Water Tank


  • Custom molded to fit neatly underneath a Ram Promaster van (159″ and 159″ extended wheel base models only).
  • Large, 26 gallon capacity with very minimal impact on ground clearance.
  • The easiest way to add a high capacity grey tank to your Promaster!
  • Includes tank, mounting brackets, 3″ dump valve and all hardware to install – only one hole needs to be drilled. See installation notes below for details.
  • Limited quantities available in our first batch!  We encourage you to order soon.
  • $80 flat rate ground shipping to the lower 48 states.
  • Additional 1-1/2″ female NPT, 3/4″ female NPT or 1/2″ female NPT fittings can be added (spin-welded onto the tank) using the options below.
  • The first batch of tanks were patched in the rear upper part of the tank and passed a leak test.  We have slightly discounted these tanks for this reason however they work as intended and the patch is not visible from anywhere unless you crawl under the van.
  • We have a limited supply of these in our first batch that we anticipate will sell out quickly so we encourage you to get yours soon!

5 in stock

If you need an inlet/outlet in a specific region of the tank, we can spin weld that fitting onto the tank for you with an additional cost. You will need to provide us a drawing of roughly where you want the fitting added. Adding this option will take 2-3 business days to the lead time before shipping. Your choices for size of fitting are: female 1-1/2" NPT fitting, female 3/4" NPT or female 1/2" NPT. Please contact us with any questions about this BEFORE placing your order.

  • Options Price: $0.00
  • Product Price: $499.00
  • Total: $0.00
SKU: VLO-PM-GREY1 Category:


Mounting a large capacity grey water tank underneath a Promaster van is a real challenge (we’ve been there). The “cavities” under the van are odd shapes and not very deep. So, we’re very excited to offer this 26 gallon, custom molded grey water tank system that was designed specifically for the Promaster!


  • Holds 26 gallons of grey water while virtually disappearing underneath the van and maintaining the ground clearance of the vehicle.
  • Complete kit comes with everything you need for the install including the tank itself, heavy duty steel mounting brackets, stainless steel attachment hardware and a 3″ gate-style dump valve assembly.
  • Mounts underneath the passenger side slider door (see illustration for details). The 3″ dump valve is located just in front of the passenger rear wheel well and can be used with a standard, flexible sewer hose when emptying at a dump station.
  • Heavy duty, 1/8″ galvanized steel (painted matte black) mounting brackets use existing holes in the structural members of the van to accept provided rivnuts – only one new/additional hole must be drilled.
  • Custom molded from medium-density polyethylene (MDPE).
  • Each of the “high points” on the tank have a 1/8″ brass breather vent to allow the tank to fill completely.

Seem Expensive?
We understand this might seem expensive but it’s a custom molded tank that is produced in small batches for a pretty niche market. Also, consider the costs of using an 0ff-the-shelf grey water tank as outlined below.

  • $182 for an off-the-shelf grey tank. There are not many grey water tanks that can fit under the Promaster and hold about 26 gallons. The one Zach used in his first three vans works alright but it is taller than our custom tank so it eats into the ground clearance more and you can see it from the side of the van.
  • $20 for a gate-style dump valve.
  • In 2019 Zach spent about $85 on the hardware to mount the tank under the van (angle brackets, bolts, nuts, etc.). Plus he had paint that all black and struggle under the van for much longer than was comfortable.
  • $25 for miscellaneous PVC parts/glue/etc.
  • Shipping can vary depending on where you source all these parts but let’s assume that it costs you $20 in shipping or gas driving to the hardware store.
  • This totals $352 in baseline costs. In addition to that, you’re going to spend a lot more time and, ultimately, if you want a large capacity tank, it’s going to “hang down” lower than this custom molded tank.

Why did we put this on the passenger side of the van when most RVs have the tank dump on the driver side? Primarily because the driver side cavities under the van on a Promaster are even smaller on the driver side because of the exhaust system. We wanted the highest capacity possible without compromising ground clearance.


The tank ships as part of a complete kit to assist for simple installation. The only things you’ll need that are not included in the box are clear silicone sealant and a rivnut tool, such as this one, to install rivnuts which are used with supplied bolts to secure the tank/brackets to the van. There are 3x brackets (front, middle and rear). Each bracket has two bolt locations (one on each side of the bracket) for a total of 6x bolts. 4x of the supplied rivnuts can be installed into existing holes underneath the van in the floor structure. One of those existing holes must be enlarged. One of the bolt locations has a factory threaded insert (M8). The final bolt location requires you to mark and drill a 7/16″ hole for last supplied rivnut.

One the tank is mounted, you’ll need connect the “dump valve assembly” to the 3″ outlet. We recommend you coat the outlet with clear silicone, insert the dump valve assembly and then tighten on the included 3.5″ hose clamp to secure the assembly.

The last step would be to plumb your drain lines into the 1-1/2″ female NPT inlet that is located on the driver side of the tank near the “front” of the tank. You can use various tee fittings to allow more than one fixture to drain into the tank.

There are 1/8” brass breather vents at the top of each of the high points of the tank to allow the tank to fill completely in these areas. If the tank is overfilled these will also allow small amounts of gray water to escape the tank onto the ground. Please be careful not to overfill the tank. Overfilling the tank may result in gray water backing up into your fixtures or vent system. We highly recommend using some kind of tank monitoring system.

Full Disclosure: Functional Not Beautiful

Beauty is in the eye of the beholder but, we’ll be honest, while we’re in love with this tank, it’s not going to win any beauty contests. It’s a rototational molded tank with a really weird shape so many of the profiles and edges are not that smooth. To be clear, it fits and works like a champ (hence our love), but it’s just not pretty. That said, it’s underneath your van and can barely be seen unless you crawl under there (to admire it as we’ve been known to do). Also the tanks were manufactured with one extra fitting on the rear top that would have interfered with it’s placement. So, we removed that fitting and patched it by plastic welding in a piece of the same HDPE material. We then coated our patch with marine sealant and flex seal. Each tank has been tested to be water tight after the patch but this is another bit of “character” on the tank.

Installation Video


Location Of Tank Under Van

Additional information

Weight 723 oz
Dimensions 56 × 10 × 23 in