2012 3950 fresh water problem - help!

I apologize if this issue has been covered elsewhere in the forums. I just don't have time to search because we are supposed to be on the road Saturday morning.
This is what has happened. We are hooked up to 'city water' and found after several days that water was leaking from under the rig, so much water that it collected in the underbelly cover and caused it to sag. Upon inspection my dh discovered that the hose between the two fresh tanks was leaking at the clamp and thought that it was just overtightened and cracked during construction. He replaced the the hose and clamp and thought the problem was solved. Then, today, he noticed that the fresh water tank was reading full at the monitor and, since we had not filled it, went to investigate. When he removed the cap from the fresh water fill line pressurized water shot out at him!??? We immediately disconnected the city water line in order to use some of the fresh water tank with the water pump. Now, while no one is using the water in the rig, the pump occasionally comes on. Any ideas as to what the problem could be?? We are suppose to start a work camping job in a few days and really don't have time to run this somewhere for service.
ANY help much appreciated.
Blessings,
Tina

Tina, there have been some owners having problems with the water pumps. Specifically, the pump allows water to back fill into the fresh water tank while hooked to city water. This might also be the cause for the pump to come on periodically. It might be loosing it's prime back into the tank and just runs to build pressure.
Best solution, replace the pump with the basic ShurFlo 2.8 pump. It's the standard die hard pump offered by them. You can have your dealer replace the existing pump but even the replacement may have issues. Or it may not. Again, the basic 2.8 is a die hard and works well. They are available at most RV dealers for under $100. Call ShurFlo. They will probably send you one along with a return label for your malfunctioning pump.
The water in the underbelly may have been from a tank vent/overflow.

We had a similar issue but it ended up being caused by the hot water tank building pressure causing the pump to spin backwards. If you talk with ShurFlo the first thing they will ask is if you have a check valve installed on the feed side of your hot water tank, in no they won't warranty the pump. The pumps are designed to support pressure from city water, however the pump becomes the weakest link if the hot water tank pressure relief valve sticks or the pressure is spread throughout the water system. Two years ago after filling the tank twice we installed the Sharkbite Check Valve http://www.sharkbite.com/usa/en/pro...s/check-valve?page=shop.browse&category_id=13 which easily connects with out tools other then something to cut the pex and have not had any further problems.

Good luck,
Jason