Heat strips are little wire elements that heat up when you apply electricity to them. They're resistance heaters. RV heat strips are limited to 1800 watts, which is something like 5500 BTU. RV propane furnaces typically put out something like 15,000 (truck camper) to 30,000 BTU (full size RV).
RV heat pumps are the same as what you might have in your house. They don't work very well below 45F, and need electric heat strips or a furnace to thaw the coil and provide supplemental heat. They suffer from the same wattage limitation that stand-alone heat strips have, so they're usually backed up with a propane furnace.
