RV Repair: 12v Electrical problem, class a motorhome, ford 460

Question
Bill, I recently purchased a 1995 Four Winds 5000 Class A motorhome and am new to RV'ing.  Everything worked fine for the first week up until yesterday.  Now none of the 12v works, the interior lights won't come on and the generator won't start (no power to the switch that cuts it on, I guess). The Ford 460 starts and runs well and there does seem to be some electrical working(the engine starter. the electric steps, the dash lights and headlights).  Can you help me trace the source of the other 12V problems?

Answer
Hi Jim:
There is either a battery isolator diode or a relay that charges the house batteries that is not working. Most of the time they are located close to the house battery compartment. When you find one or the other check the voltages at the teminals with a voltmeter it will tell you where the problem is.
Bill