Water holding tank problem

[5thWheeler]

It must be camping season again since there are several new threads and problems to fix. My current problem is with my water holding tank on my 5th Wheel trailer. It is a 50 gallon tank and when I fill it about 30 gals drains out of the overflow tube that is on the bottom of the trailer. I am not sure if the overflow tube is tapped into the tank on the top or the side since I didn't remove the underbelly sheeting. My first thought is that there is a siphoning effect taking place but I am not sure why. It is interesting to note that when the water is draining out of the overflow tube there is air being sucked into the fill nozzel where I attach the hose to fill the tank. So now I am thinking that the water tank is being pressurized somehow by drawing air from the fill nozzel. To confirm this I installed the rubber dust cap on the fill nozzel and the rate of water flowing out of the overflow tube slowed down significantly but didn't stop altogether.
To allow us to use the camper this weekend I installed a valve on the overflow tube to keep the water in the tank but I realize this is not the best solution because if I forget to open it when filling the tank, the tank could rupture.
Does anyone have any ideas as to what may be causing this problem? The water pump and all of the faucets are working fine.

Thanks in advance for any ideas someone may have.

5thWheeler

 

[LEN]

I think the overflow tube is too long in the water tank. So when the water is full enough to start the siphon it will do so, then siphon down to the tube depth. Only way to cure the problem is shorten the overflow hose length in the tank to the correct height.

LEN

 

[Grandview Trailer Sa]

5th Wheeler, what you have is a venting problem. I have seen vent hoses pinched and the the tank swell up and push the water back out of the fill hose again, but not exactly what you described. The hoses do not extend into the tank...they attach to the tank with fittings and clamps.

 

[wildcatervin]

5th wheeler,Had this problem with my RV.Mine would siphon the tank empty going down the road.Took it back to the dealer and they added a hose to make it go above the tank .Mine was easy to get to.And they do attach to the side of the tank.Just get a longer hose and make it go above the tank and then out the bottom of the RV.Solved my problem.

 

[LEN]

If the tube is only attached to the tank at a given level, when siphoning it would only go to that level and stop the siphon. I do agree on the venting probem, but this one appears to be two problems. A picture of how the tank is built would sure help.

LEN