My trailer is plugged into a 30 Amp RV plug in its own subpanel that I installed outside my garage door, but it is only fed with 12 gauge Romex wiring on a 20 Amp breaker from the garage main panel. I have no problems running the A/C, the fridge, and the TV set on 120 vac, all at the same time. You should be fine with a 30 Amp circuit.
I agree, I have installed a 30 amp RV outlet in a weatherproof box just outside of our garage door. The run from that box to the main breaker panel is about 20 ft. I used some #12-2 w/ground to a 20 amp breaker in the box. That installation will run everything in the Casita with no issues.