I'd think also that a space heater set on a low temperature should be fine. Also, perhaps a 75 -100 watt light bulb in the wet bay to keep things cozy there. In the future, you might invest in a cap for the water fill that has a air fitting in it. That way you can blow out the water lines and not have to mess with RV antifreeze and flush to recommission. Don't forget to leave cabinet doors under sinks open to allow heat into those spaces.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI