eyeteeth - regarding your "sometimes it does, sometimes it doesn't" starting mystery ... interesting, I wish you luck finding out why.
(Sorry, I couldn't resist a bit of sarcasm ... it sounds like one of those situations that causes hunched shoulders and flat foreheads.)
It's an intermittent malfunction ... which tends to be especially frustrating to troubleshoot and fix.
I'd start with your ignition switch. It may be worn enough to cause intermittent faults. Or, the wire connections on the back of the switch may be loose or otherwise faulty.
As always, a loose ground connection could also be the issue. Try jumpering around the battery negative/ground cable and wires.
Finally, the starter relay may be malfunctioning or its wires and connections may be loose or faulty.
BWD part number S654P. CarQuest list price $20.99
Dang it, folks, I'm starting to think I need a list of what everybody has, in terms of motorhome year, make, model, chassis, engine, and so forth. (My CarQuest commercial account keeps track of recently referenced vehicles but I can't remember which ones belong to who.)
