Using the 20 amp plug, or 15 amp in some place, with an extension cord to an appliance is okay as far as I've been told, working as a host or as a camper.
The Texas State Park were were hosts at wanted the duty Ranger, and the Law Enforcement Ranger notified immediately if we saw a device plugged into the site to combine the two plugs into one input for the RV. I've seen tents, tear drops and popups with no power wiring running heaters from the 20 amp plug.
The worse abuse in my opinion is when someone gets a spot with two 30amp pedestals for two sites very close and plugs in a cheater to use the electric from both sites to make 50 amp for his rig.
Even had one charming gentleman tell me at one COE campground that because he got their first, I had to camp without power.
Went to the gate attendant and wanted a new site. Got one, and watched his Newmar being escorted out the the campground about two hours later by a COE Ranger and a county sheriff's vehicle.