The ATS (automatic transfer switch) will sense the power from the generator. The generator power is "priority" over incoming shore power. Therefore, the ATS will switch to generator power.
But, when I test my generator, I turn the shore power off, then start the generator. Like it would be if you lose power.
On our Road Warrior there is a switch in the control panel area, where the light switches are located, that is a red color switch and when turned on it lights up. That is our tank heater switch. There is another switch, "Yeti Package" which is for the water line from the freshwater tank to the water pump.
DO NOT turn on if you do not have water in the holding tanks, you could damage your tanks.
If you are going to get a hard freeze, you may want to run your furnace to keep the water lines from freezing up.
If you have low point drains, you need to prevent these from freezing as well. If these freeze, the ice can go upward into your water lines and cause a blockage of water.