I have a leak in my 5th wheel Scamp that I am having difficulties figuring out where it is coming from.
Given the volume of water you mention, the culprit is probably your fresh water tank or plumbing, somewhere between the water outlet on your fresh water tank and the pump. (If you don't have a pump any fitting or connection below the level of the top of the water line in your tank is suspect.) The weakest link in the system are the points where two or more pipes or hoses meet at a fitting, secured and made water-tight using a hose clamp and the fitting that connects to your fresh water tank to the rest of your plumbing. Another possibility is there's a crack in your fresh water tank or a leak in your pump housing.
Here's how to troubleshoot the system: Empty your fresh water tank and clear out the areas under the dinette bench seating area and under-counter cabinets and get everything dry. Once everything is dry, locate all the fittings where pipes connect to one another or the fresh water tank and put a dry paper towel underneath. Use a rubber band to secure paper towels to the pipe fittings that connect your fresh water inlet and tank. Fill the tank and check the paper towels for dampness every few minutes.
If you find a damp paper towel at a single location where pipes come together, try tightening the hose clamps at that location, dry things up, and try again.
Pipe clamps can and should be made really tight. When you use them to secure radiator hoses in cars the material of the hose bulges through the little slots in the band of the pipe clamp.
If you get two damp paper towels, there may be a leak in the hose between them. Replace the hose with clear vinyl 1/2" hose with an embedded reenforcement mesh in it. (From Lowes by the foot or at Home Depot in 10' reels).
If the leak appears at the water tank and tightening the hose clamps doesn't work, you may have a crack at the fitting where the hose connects or in the tank. There are a couple of ways to fix this, but the best solution is to buy and install a new tank with the fittings already installed. (I found the fittings I installed in the 20 gallon tank I bought without fittings
leaked, and had to invent a way to retrofit fittings into my new fresh water tank. I'm still in the wait-and-see stage of things to see if my solution works or I have to buy a new tank.)
If the leak appears at the water pump and tightening the clamps doesn't work, drain eerything and try unscrewing the nut-like fittings that connect the hoses to the pump (you'll have to undo the pipe clamps, too) and wrap the threads with plumber's teflon tape -- two wraps only without covering the hole where the water goes! -- and put everything back together, nice-and-tight, again. If that doesn't solve the problem you may have to buy a repair kit with new gaskets and diaphragms and re-build the pump housing or replace the pump.