If you have hookups go with a electric oil radiator type heater. You can set th thermostat set your LP furnace a little higher as a backup. I have not camped in the 20 but have been in the 30's. We also use a heated mattress pad.
I was wondering if you decided to go with the Closed cell board if it would just lay on top of the black fabric material assuming the material can be taken down witout cutting... The board hardly weighs anything.
2017 Coleman 300tq by Dutchman Toy Hauler. 34.5 feet long and under 10k Gross. 1999 Ford F250 2WD 7.3 4R100 DP Tuner, S&B Cold Air Intake, Gauges, 6.0 Trans Cooler, Air Bags.
You must NOT have A/C...If only two wires for heat only. I bought 2-inch foam sheets and cut them to fit all of the floor spaces between the aluminum sub floor. Also, cut 1 inch foam panels for your roof vents and windows...when it get down to 22 degrees. Awesome and cheap insurance to a toasty RV.