Yes they are called cheater boxes. Lot of camp grounds will not allow you to use them. I have one and you are correct. They work really good with two 30 amp outlets.
I always worry about voltage in some of these places. Low voltage can ruin an electric motor faster then what a lot of people realize. Most of the time the circuit breaker will kick off, but a lot of times the damage has already been done.
My coach has big units with heat pumps and it will not run two air conditioners on 30 amp. My coach was designed for 50 amp and that is what I try to use most of the time. When only 30 amp is available I only will run one air conditioner, and if it is too hot, I will just drive off and go to another resort. Never have had to buy a unit yet that is combo air and heat, but I imagine they are not cheap.
2002 Gulf Stream Scenic Cruiser 38'
Neway Freightliner chassis, 2018 Buick Envision Ess.
3126-E Caterpillar 330 hp