Water leaks are major pain to diagnose. However here are a few things I have tried. First, get an inspection camera with a fiber optic camera. Harbor Freight Tools has some inexpensive ones that work well. Use a water hose without a nozzle and start at either the front or rear. Keep in mind that leaks may take awhile to show up. Be patient. While running water open the cabinets doors etc. and use the inspection camera to look for telltale signs of water intrusion. Depending on the sidewall construction pay attention to anywhere anything is attached to the sidewall including the trim pieces etc. Don't use high pressure to test with you just want a lot of running water on a selected area you are testing. A thermal camera can dramatically help. Also keep in mind that water may be dripping on the inside on one place but actually leaking in in another place. Sometimes water will be leaking in an upper sidewall but then running inside the roof and dripping down in the middle of the coach.