At the moment all OK. The fresh water tank is under the bed, and there is a radiator in the garage, so won’t freeze. The waste tank under the van is insulated and heated, I have the tank heater on.
The heating does not drain when you drain the water down. The wet heating has antifreeze in it. So when you drain the tanks you are only draining the hot/cold water systems.
To recommission after draining down you need to close the drain valves, taps and shower, then fill the fresh water tank. You then switch the water pump back on and purge the air out. You run each tap/shower in the hot and cold positions until all of the air is out.