On my Cruiser, there is no insulation between the metal heat duct and the underbelly so most of the basement heating is via radiated heat. However, there was also a single 'vent' into the basement, it was a 1 inch diameter hole punched thru the bottom of the duct. I plugged it up. I don't want to waste propane heating the basement when temps are in the 40s and 50s since we also do much of our camping in Tx.
I frequently use electric heaters whenever I can. I have camped for several cloudy days with temps continuously between 25-27 degrees, used no propane, and never had a water problem. The first thing that is at risk of freezing is the fresh water pickup tube that feeds the 12V pump. If you are camped and leave the gray water valve open, then that is not a freeze concern. Here in Texas, if you pay attention to the forecast, it is unlikely you will need to open the black tank valve while temps are below freezing.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK