Bizarre holding tank problem! It fills up on its own!

The "calling" for water pressure is interesting, if we assume there is a leak filling the tank.

A leaky faucet does not call for water. The pressure would constantly be calling, and there would be no difference that would call for it.

A residential toilet can call for water. A leaking bowl seal would drain the bowl, lower the tank level, call for more water to fill the tank. Most RV toilets do not...you have to flush. Plus this would assume that your toilet is flushing into the gray tank...yuck.

A water heater can also call for water, but that leaves a draining/gray water tank filling question????

My pressure release valve on the water tank would just spew water on the access panel. Is it possible yours has a drain into the gray tank. Pressure builds, drains into tank, calls for water to fill tank....repeat. That is still a lot of filling/draining to fill a gray tank.

What about a fridge with ice maker/or water dispenser. A leaky dispenser is probably like a faucet, which would be a constant leak---constant pressure. The ice maker would call for water...I have no idea where they would drain to???? .

Perplexing question. You MUST post the answer when you figure it out.

Sorry, I just reread and note your water tank is not connected. Wonder if there is an issue from that.

Welcome to the forum.
I have a 96 Fleetwood Southwind. Is your Bounder equipped with a washer/dryer combo?

When you hear the water cycle on and off, is the water pump running? Or are you hearing the water flow from the city water inlet?

Do you hear it cycling only when not connected to city water?

I don't have the answer but, the more info we have will help us help you.

How often does it cycle?

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.

I would suspect that a previous owner installed a holding tank flush for the Gray tank and then installed a valve connected to the pressure side fresh water. Look and see if you find a valve you do not know what it is for in an outside storage close to the Water bay and Holding tanks. Doug

Do you have an "outside kitchen"? I had one years ago and the outside sink faucet was on and draining water into the gray tank??