When I had a toy hauler, I had to store it where it could not be plugged in. After each trip, I would make sure all the tanks were drained (use the water heater drain plug to empty it too), lines were blown out and pink fluid dumped into the drain traps. I never pumped the pink stuff through the lines because we camp at least twice a month all year.
Now that I have a TC, I can keep it at the house with electric hookups and will set an electric heater to kick on to maintain 45 degrees inside. This is enough heat that the other compartments will not freeze and nothing has to be winterized. When we take off on a trip, I leave the LPG furnace thermostat set at the lowest setting and it keeps the lines from freezing while we are in transit.
You will need to experiment with heater settings to see how low you can go without freezing your storage compartments. I use a remote thermometer to monitor storage compartment temperature and now know how to set my thermostats so they do not freeze.
I do not have indoor storage, so my RV's have to be able to handle the weather year round outside. I don't cover them because of how frequently we use them - This does mean you have to be diligent in inspecting the seams and caulking often.