In a 50A service you have 2 120V 50A legs, and in 30A you have 1 30A 120V leg. The Alfa was wired so that the extension was used to operate the 2nd compressor but that does does make it 50A service. 50A service has almost 3 times as much power available as 30A service does. I'm surprised that setup worked w/o tripping out a breaker on the pedestal since the 20A and the 30A are wired (usually) from the 30A circuit and not a separate 20A circuit.
Actually...the 30A and 20A are probably on the same
LEG of the pedestal, but they each
SHOULD have their own breakers since they are being fed from the same
BUS in the pedestal. The 20A should
NEVER be wired from the 30A circuit, but they will be on the same
PHASE or
LEG in the panel, since a 30A panel only has 120VAC possible. A reading with a volt meter from the hot of each receptacle will verify this.
In a pedestal that has 50A service, there are 2 hot
PHASES in the pedestal, each with 120VAC to ground.
And, yes, it is a requirement for all single-phase, 15 and 20A, 120VAC receptacles, located outdoors, to have GFCI protection for personnel - few exceptions. This can be in the receptacle itself, or the circuit breaker. In fact, many (newer) installations will have the 30A GFCI protected also, although not yet required. With todays RVs having larger electrical systems and demands, the rules are continually being revised.