What is going on!!!!!!!!!

Mine did the same thing and it several trips to figure out the water was coming out of the over flow hose especially when going up hill or taking corners. I just routed the overflow hose about 4-6 ins above the fitting on the tank which is on the side rather than on the top over the tank. This fixed the problem. I would not install a shut off fitting in the line. I saw a guy forget to open the valve when filling his tank and the expansion of the tank broke the attachments to the frame. It took myself and another camper all afternoon to make a FG repair and new steel angles to fix it.

I was concerned that a simple hole might allow water to escape into the insulated area when filing so I added a 6" riser and covered it with a loose cap for bugs, dust, etc.

Bruce Brown wrote:

Old-Biscuit wrote:
Just drill a 1/8" hole on high point of overflow pipe.

Yup. I've done this with every MH we've ever owned.

If possible, I extend the height of the overflow with a T first. I put a plug in the top of the T then drill a small hole in that (red arrow). I use the side of the T for the overflow. We've been camping for over 20 years now and this method has always worked. Simple, cheap, effective, and nothing to fail.

A very good home-made anti-siphon valve. Since these are on the potable water tank, I would add a small screen to the opening to keep dirt and insects out of the potable water tank.

Old-Biscuit wrote:
Just drill a 1/8" hole on high point of overflow pipe.

Yup. I've done this with every MH we've ever owned.

If possible, I extend the height of the overflow with a T first. I put a plug in the top of the T then drill a small hole in that (red arrow). I use the side of the T for the overflow. We've been camping for over 20 years now and this method has always worked. Simple, cheap, effective, and nothing to fail.

Old-Biscuit wrote:
Just drill a 1/8" hole on high point of overflow pipe.

My kind of guy. :W

Just drill a 1/8" hole on high point of overflow pipe.

CA Traveler wrote:
Plastic tanks expand when full, just install a anti siphon T and short riser to resolve. My riser is 6" long with a loose cap on the top.

Best answer. There are also plenty of anti-siphon valves commercially available.

Plastic tanks expand when full, just install a anti siphon T and short riser to resolve. My riser is 6" long with a loose cap on the top.

Plastic tanks expand when full, just install a anti siphon T and short riser to resolve. Mine riser is 6" long with a loose cap on the top.

I had a Jayco TT that did the same thing. Added a shutoff valve on the overflow line. Opened when filling and closed after full.