Jump to content

Question about filling trailer freshwater tank w/gravity from another tank

Recommended Posts

I'm investigating putting together a vented fresh water tank to carry boondocking water in the bed of my truck instead of carrying multiple 5-gallon water cans.  The bottom of the truck tank will be above the top of the trailer tank with the goal to use gravity instead of a pump to refill the trailer tank with a hose connection. To do this the trailer tank needs to fill unobstructed into the top of the trailer tank.


The Legacy II trailer tank fill hose input nozzle is located below the trailer tank.


Does anyone know how/where the fresh water tank fill line is plumbed from the input point and where it enters the freshwater tank?

Does the input plumbing tap into the bottom of the tank and fill from the bottom?

Does the input pluming tap into the top of the tank and fill from the top?

Does the input plumbing have a check value or a device that prevents back flow between the tank and hose nozzle?

My first desire would be to empty the truck tank into the trailer and be done with moving water. If the above proves to be problematic, would it be possible to use the boondocking freshwater input nozzle / pump to “draw” water directly from the truck tank with a short hose connection? If this method valid, is my assumption correct that the truck tank will not fill the trailer tank but would supply water to the trailer system as long as the truck tank is linked by a hose to the boondocking input? In this case, I suppose the shorter the hose the better.


Your knowledge and thoughts will be appreciated




Link to post
Share on other sites
  • Moderators

Typically, you would connect the truck tank to the boondocking intake and pump the water into the freshwater tank. It would fill as fast or faster than gravity feed.


You could gravity fill the freshwater tank through the freshwater intake if you wanted.


You could configure the valves to pump water from the truck directly to the faucets, via the boondocking intake.


You could not use the truck’s tank as a continually connected auxiliary, via the freshwater intake, since the water would just flow through the fresh tank and out the overflow.

  • Thanks 1

Snowball • 256 • 2018 Ford Raptor

Link to post
Share on other sites

As overland said, there are a number of options, but connecting a 3/4" garden hose to the bottom supply port on the truck tank with an inline ball valve to the winterization port would be simplest and most effective. Unless there is already a ball valve incorporated in that tank.


You could leave the hose hooked up all the time. When the trailer tank gets low, open the ball valve and use the Ollie's pump to refill the tank - you can easily position the correct valves to pump it into the fresh tank. When the overflow spits water, close the ball valve and reset the onboard valves. Repeat until your truck tank is empty.


You need a good sized hose since it will be on the suction side, but the tank head pressure will assist, assuming you are not parked downhill.


For a nice looking and sanitary setup, considering using these scrumptious Eley hose accessories with solid brass quick detach fittings. ...  They have a 3/4" inside hole, unlike most other fittings which are quite restrictive.




... https://www.eleyhosereels.com/collections/watering-tools


John Davies


Spokane WA

  • Thanks 1

"Mouse":  2017 Legacy Elite II NARV (Not An RV) Two Beds, Hull Number 218, See my HOW TO threads: https://olivertraveltrailers.com/topic/john-e-davies-how-to-threads-and-tech-articles-links/

Tow Vehicle: 2013 Land Cruiser 200, 33" LT tires, airbags.

Link to post
Share on other sites

To answer your original questions: The fresh water fill line enters the tank on the back side near the top right corner. This line does have a backflow check valve hard plumbed at the water inlet. This check valve has a 2-3 psi crack pressure that would have to be overcome, so gravity feeding through the fresh fill inlet might be difficult.


Depending on the height of your aux tank, it might be possible to gravity feed through the boondocking inlet and backfill the tank through the pump pickup tube.  There is no backflow prevention in this path. You would just need to open both valves on the left side of the diagram. But again, the height of the aux tank would be key and need to be higher than the plumbing elevation.


I'm onboard with the others that suggest just treating the aux tank as boondocking source. To me, it seems better to transfer the water to the main tank rather than demand pumping from the aux. The water temperature will be better maintained and you won't have to disconnect and reconnect every time you want to move your tow vehicle.

  • Thanks 1
Link to post
Share on other sites

Thanks to all for sharing your knowledge and coaching. In case you are interested, I have found a company that will make up customized water tanks. They will install the ports to your liking (type of port and locations)  in a selection of various tanks. The turnaround time is 7-10 days and the cost including freight is very reasonable.  Zach is the tank expert and a great guy.








Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...