That looked to be a red flag when you said that you used three space heaters. The furnace ducts running under the camper are there with the belief of keeping the temp level above the freezing temp. I believe this would be correct if you were only using the furnace and not space heaters. You would burn the heck out of propane. getting warm air going through the ducts would help so much to keep any lines from freezing. I like the idea of setting a small space heater in the outside kit.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler