The electrician made an error when he used 12 AWG wire and a duplex outlet for a 30amp circuit. 30 amps requires 10 AWG wire minimum for a 30 amp circuit.
Here are the wiring requirements:
15amp - 14 AWG
20amp - 12 AWG
30amp - 10 AWG
Also, a 30 amp circuit requires a different receptacle compared to your normal 15/20 amp duplex receptacle.
If it were me, I'd connect a new 30amp breaker, 10 AWG wire and dedicated 30amp outlet for the RV, and I'd replace the 12 AWG wire and duplex outlet for the compresser.