OK

Understood. First the water pump should be turned off. The water pump will take water from the fresh water tank and pump it through out the RV. When hooked up to a city water source you do not need the pump, there should be adequate pressure from the incoming source. A reason the pump could of been running is if your fresh water tank is empty or it could of been trying to recirculate the city water. IF this pump was running not being hooked up and the fresh water tank was full I would be concerned about a leak, this sounds like it is not the case.

Now the Black and Grey water. The only thing that should go into the black tank is your toilet. All your sinks, and shower go into the gray water tank. The reason it would back up is blockage in the plumbing especially the p-trap. Also the shower normally will back up first when the gray water tank is full. In your plumbing bay is the handles for the black and gray water dump valves. Sometimes these valves will actually break and even though the handle is out the valve portion is stuck shut. This is why it is important to understand your tank levels. IF you gray water tank is reading full, the valve is stuck shut or the holding tank in the ground is full. (also when the sensors get old and dirty they will give false readings) One thing you can try is to get the water down and try using equal parts baking soda and white vinegar,put the baking soda down the drain followed by the vinegar. Depending on how bad the clog is, you may need to do this more than once. Also you can try very hot water after adding the vinegar. Adding liquid draino or a similar product can do damage to various seals and may sit in the tank and cause fumes.

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