The switch in the stairwell is the battery disconnect for the coach (MH) batteries. It is turned off when MH is not is use to save draining the battery with things like LP detector, etc., that are otherwise always "on". When occupying the MH you will want this switch in the ON position to provide 12 volt power to coach (lights, refrigerator control board, furnace control board, thermostat, fans, etc.)
Also when that switch is ON, running the MH will charge that battery. There is an emergency start switch on the side of the driver's seat that connects that battery to the engine to start the motorhome if it's own battery (chassis battery under the hood) is dead. You press and hold the emergency switch while starting the engine.
All that said, the condition you describe should not happen. The battery disconnect switch in the door step should have no effect on the starting of the MH engine, except if you are holding the emergency start switch. So, if that was the case (engine wouldn't start unless disconnect was on), I would suspect that something is wired very incorrectly. If the problem can be duplicated, I would suggest getting it back to the dealer for a complete inspection and diagnosis.
In normal operation, you will want the battery disconnect in the on position at all times that the MH is in use, and will turn it off when the MH is parked (stored) without shore power attached.
2012 Forest River Sunseeker 2300 Chevy