Many RV dealers stock HW heater winterizing kits.
These are a bypass for the the HW heater so it can simply be drained , and the HW system filled with pink from the water pump..
Next year , simply moving 2 tiny lever valves and you are all set.
Beware , heating the pink stuff may cause it to solidify a bit and then be really difficult to flush out.
It will also solidify if in contact with air , so tossing it into a water tank, just in case everything may not have drained , may require living with pink water as repeated flushing only thins it out.