We have a 1987 motorhome which is experiencing battery drain problems. There are 2 house batteries and 1 coach (start) battery. All batteries were new. Nothing shows to be actively on including the electric step. However, the batteries are completely drained within three or four days unless hooked to electricity. We had to change out the ignition switch. I don't know if this could be part of the problem. How do we find the problem?