I drain after every trip, and leave the taps open. Just in case I leave a fan heater on set to keep about 4 or 5 degrees, it’s worked for me so far.
I carry some salt and a small shovel, and replace the summer chairs with much lighter ones. The summer duvet is changed for a winter one. When out in the winter it’s always toasty warm in the van, I leave the heating on at night.
After every trip the van gets a thorough wash to remove any road salt.