Wiring one air conditioner to use 20 amp outlet

The simplest way is to go to the first junction box out of the genset, determine which set of leads power the rear air, and install a pigtail with male and female connectors on them- leave them plugged together for genset power, plug it in to a cord for shore power.
This does mean you are relying on the park breaker for circuit protection, and there is a kit out there that includes a breaker (someone will come up with the name).