[tentcamper]

No the fresh water holding tank has it own drain valve. The water in the tank is isolated from the pluming withing the camper by the pump which has a one way check valve.

It sounds like in the rear you have what I have.

What I have up front is a cold water line drain valve and a fresh water tank drain valve. I'm wonder if what your looking at that does not have a valve and you think may be an overflow, may be the fresh water tank drain. The valve was either knocked off or the valve is inside the camper at the bottom of the tank and you'er just seeing the drain tube.

I would follow the tube up into the camper and see where it go's. You may need to add a valve to be able to fill you fresh water tank?