It takes 2 stages to get the gas down to the pressure required. The first stage lowers the working pressure to a range that the second stage can regulate to the approx 4 PSI that the trailers use. Going from the extreme pressure of the tank to the low pressure of the appliances would be difficult with only one regulator. In a BBQ it does not really matter if it is off by 5-7 PSI. If the pressure is not just right the appliances in your coach will not work right.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato