Water leak at fresh water opening - HELP

We took our 2007 TM 2619 out for the first time this weekend and need some advice.

I was testing most of the systems to ensure that they worked and ran across the following plumbing issue.

1) I had the fresh water tank full with the water pump in the OFF position.

2) I connected the city water to the campsites water faucet and have a copper 40 - 50 psi regulator inline at the connection.

3) I ran the air out of the lines by running the kitchen, bathroom sink and tub.

PROBLEM:

I had a considerable amount of water leaking from the fresh water fill connection (The one directly above the city water connection on the outside of the TM) although I was not using the fresh water/water pump system. I would unscrew the plastic fill cap and water would gush out at a good rate until the pressure normalized and the water level dropped below the fill line level. I would screw the cap back on and within minutes the same event would occur.

ATTEMPTS TO FIX:

I have ensured that the vent/overflow line on the fresh water tank is unkinked and working correctly. I have tried my home water line with the same regulator in place.

The problem still persists.

The amount of water leaking was enough to warrant us turning off the water at night so as to not completely saturate our campsite and the neighboring campsite.

IDEAS?

One last note:

I read the shurflo water pump technical sheet and it has a diagram of a "typical water system" which shows both the city water and potable holding tank systems together. I notice that on the potable tank system there is a shut off/drain valve prior to the intake of the water pump.

I do not have a cutoff present in the plumbing system of my TM...

Should I and Do you?

I figure a cutoff valve would stop the city water pressure from entering the potable water tank and avoid the problem but I don't want to cut into the line (the connection between the fresh water tank and water pump is flex line NOT pex) if it is going to damage the plumbing system in any way...

Thanks!!!