If it is starring with the boost switch, then the starting circuit is in tack and working normally. As others have said, start at the battery, check voltage on the battery posts, then posts to cables, then cables to the battery control box under the hood on the bulkhead. BE VERY CAREFUL AS THE BOX CONTAINS THE BATTERY VOLTAGE FROM BOTH CHASSIS AND HOUSE BATTERIES. A short to the box will very likely cause whatever causes the short, to explode. There are relays in the box and may be some near it for the chassis battery.
Good luck and let us know what you find.
