Can't say that I have used my camper in those kind of temps but when it get's down to 20F-25F, I use the propane furnace to get the interior temps up to about 65 and then use an electric cube fan heater (from Home Depot) to maintain the temps. I keep a second electric cube heater to supplement the first when it gets really cold.
Each heater is rated at 1500W, so you can do the math and figure out how many heaters you will need.
When we are boondocking, I use the small propane "coleman type" heaters to supplement the furnace. They take the chill out first thing in the morning.
You can always put on an extra sweater !!! works every time.....
gerry