Wow, I don't know what happened but part of my reply is in Steve's quote. It should have been:
That is correct, meaning that you can't go through two regulators, the RV and then the grill.
If you put a "T" connection in I would place ball valves so I could shut off the side not being used.
Also, an employee at a propane store, for liability reasons, is more than likely not gonna tell someone off the street that he/she can start tampering with a factory installed propane system.