We stayed in the Boise-Meridian RV resort in Jan/Feb several years ago. It does get cold, snow was blowing sideways in front of the motor home
! Be very thorough in your winterizing. I would consider also getting a large (100lb) propane tank. Quite possibly the park will loan/rent the tank?
Here are a few things you may or may not have considered:
Foam inserts for vent/fan openings
Tank heaters
pre-assembled heated water hose
extra portable electric heater (word of caution here: don't use as only source of heat - you need the furnace to operate to keep heat flowing to the tank area)
Good luck in you adventure
!