[folivier]

Larry I installed this one over the winter. My air compressor didn't have an auto drain and the old one was leaking. I read about the solenoid drain and found this one. It works good, doesn't need power, and is cheaper. When it fills up the float opens the drain and it dumps water automatically.
http://www.amazon.com/gp/product/B0078SE...UTF8&psc=1

Read through this thread, it describes the solenoid dump and also describes where/how I mounted mine with pictures.
http://newellgurus.com/Thread-Auxiliary-...-Separator

Also reading your post again, do you have a check valve downstream of the air filter? If not or if it is leaking adding a check valve will keep air pressure in the coach system from releasing when the solenoid opens to dump water.