I think this is more of a standard issue and not NEC. The requiremnts is that all fluids be located in a separate enclosure away from electrical componets (AC or DC). The exception is that if an electrical device is directly a part of the Valve, solenoid or regulator controlling the Fluid it is acceptable to be in the same enclosure.
So your right, Gas is considered a fluid,it is not allowed. As a matter of fact pneumatics are considered a fluid and also are not allowed. Your other issue is if the LP gas is volital, all the electricals if located in the same enclosure would have to be rated for explosive atmosphere and the enclsure would have to be rated for explosive atmosphere. This would be big big money to design an enclosure for that type of application.
Christopher Caserta
ccaserta@us.tuv.com
Ph:904-225-0360