Since you haven't told us when the engine doesn't start, if it turns slowly, as if dragging or if it turns normally but doesn't fire, I'll assume it's dragging.
Pull the house batteries and have them tested. Sounds like there is either a shorted battery or something in the house wiring that is putting a HEAVY drain on the batteries.
While the batteries are out, try to start the engine, if it starts with no problem, I'd suspect the Isolator. As you reconnect the batteries, see if you get a heavy spark when touching the terminals with the cables.
The battery "Boost" solenoid is a standard starter solenoid activated by pushing a switch on the dashboard. If you hear a loud click when you press the switch, it's probably working. Locate it in the engine compartment and take a good look at it. If you see any burning, rust or corrosion around the heavy terminals, it might be grounding out. If you don't hear the click, look for a blown fuse.
Have a good one,
Hey! How's that "HOPE & CHANGE" working out for you?
January 20, 2013 - The end of an ERROR!