I believe that the Battery Disconnect switches above the side entrance/exit door completely isolate the batteries. If switched OFF the batteries will not supply any power or be charged. My 1997 Fleetwood Southwind owners manual says to turn the batteries off if storing the motorhome for more than 10 days (to prevent small currrent draws from draining the batteries). It also says "When charging the house batteries, the battery disconnect switch must be ON". Having no 12 volt from the house batteries to the motorhome and no starting power to the generator are definitely related, the house batteries are used to start the generator, not the chassis battery. I found this out when my furnace drained my house batteries during an extremely cold night in Texas. The AUX START switch on the dash allowed me to start my generator with the chassis battery. You say that you can hear the relay when operating the MAIN BATTERY switch, but nothing when operating the AUX BATTERY switch. To me, this suggests a problem with either the AUX BATTERY switch, the AUX BATTERY relay, or the house batteries (maybe the batteries are totally dead and will not operate the relay?). I'd check the house batteries, at the batteries, for 12+ volts first. Remember that you most likely have (2) 6 volt batteries connected in series, the positive on one battery will be connected directly to the negative on the other (with a short battery cable). Test the voltage across the two terminals that are not connected directly to each other, one on each battery. If you get a low voltage reading, charge the batteries (after checking the acid level and topping up with distilled water if needed). You may be able to charge the batteries by plugging the motorhome into 120 volts and then operating the AUX BATTERY switch to ON (you should hear the clunk of the relay, the same as the MAIN BATTERY switch) If the house battery voltage is 12+ volts, and the AUX BATTERY switch does not cause the relay to "thud" you will have to investigate the switch and relay. Sorry, I don't know if there are any fuses in the system or where they would be.