I use a small electric 1200w heater for maintaining temps in my 24'. Since I have the 24' TT super-insulated, comparative use isn't possible but an electric heater of 1500w should keep you comfortable in temps as low as 20f overnight. It won't do much to keeping water lines and tanks unfrozen at these continual lower temps.
Depending on how your 20' is insulated and your comfort level, a 20 lb. cylinder should last at least a few days and maybe more at a continual 20f or below.
I've taught myself to crack open a vent at night if a propane appliance is used. Your OEM furnace is vented to the exterior but one can't be too careful.
