No good way. You just have to check and seal everything. The marker lights are known for leaking. One of mine was. Leak behind the light where the wire goes into the shell. You basically have to take the light off and seal behind it and around the wires. Roof leaks eventually end up on the floor. The rub rail around the belly pan can leak. If the PO sealed around the rear window I would sure check that area again. I have found small holes in the roof where there is no logical reason for a hole. Look in the compartments at the the rear for water.
Could it be a pipe leak? Lot of plumbing around the area of the water inlet at the rear. Check valve, pressure regulator, Y's, stuff that can leak. Could leak there with either city water hooked up or using the pump. I would pull up all the carpet and the carpet in the compartments and try to dry it out.