well, previous topic dealt with my house batteries. turned out old ones were dead. replaced both, all was well. So day before heading out for Oregon i do some prep. began by retracting leveling jacks ...
The only thing that I can think of that will affect both the house and chassis voltages is a short to ground by the battery isolator. This would prevent the alternator from charging the batteries and providing enough power for the ignition. I'm guessing that it is not a permanent short or the engine would never start but something that fails after a period of time and shunts all of the current to ground instead of charging the batteries.