All water coming into your coach should be going through the whole house filter. The water should then be redirected to either the holding tank or to be used directly. If you can fill your tank but not use the water directly I don't see how it could be the filter at issue. I would be considering that the diverter valve must be the issue. Is there also a winterizing valve somewhere? Maybe this is causing some of the problem. Water heater by-pass valve? It just seems like it must be a valve somewhere.