It is not a good idea to transfer loads HOT. but that said this applies mostly to Air Conditioners and Water Heaters (And space heaters) 3 things you should NOT be running on a 15 amp outlet at the house of friend or family.
If you are really concerned do this.
Shut off A/C if running, trip main breaker (turn off manually) start generator, wait for it, wait for it CLUNK of the transfer switch, reset main breaker.
To switch back, same procedure.. Only the transfer switch drop out will happen as soon as the Genny stops.
Resistive loads (water heater, RV Fridge) do not need a 'Cool down'
Electronic loads (Converter, Television, and all electronics) need less time than it will take you to get from the Generator Kill switch to the main breaker.
Compressor loads (Air conditioner, residential fridge) need 3 minutes.