While hooked to city water only I wonder how you hear the water turning on and off. I assume you are referring to the pump but of course it should not be on. The only other thing you would hear would be a valve at one of your sinks opening and closing and that would be weird but also hard to hear. So start by unhooking from city water and running soley off the on board with pump. If the pump turns on and off you know you have a leak and if it drains yourfresh water tank and backs up the grey into the shower you know it is finsing its way to the grey tank. Now how is the mystery. If the onboard system does not produce this result then it is exclusive to the city water. Do you have a pressure gauge attached. I have now left my comfort zone but think you need at least to isolate the problem. I will be interested to know the outcome but am sensing an overflow into grey because of high pressure but just guessing. I don't know that much about these older coaches other than that the plumbing gets brittle and breaks. Good luck and let us know when it is solved.
