It can be done. Click on this
LINK.
I modified my 30A service to add a 15/20A power cord so I can run both Air Conditioners from shore power.
I've been using it for several years and it works well. I had problems with the 20A circuit breaker. Even though the A/C draws about 10A when running it would eventually trip the circuit breaker. I have changed to a T20 fuse. The fuse has NEVER blown.
See the attached updated drawing.