An RV furnace is a built-in, non-portable heating solution for RVs. They typically run on propane gas, but electric alternatives are becoming increasingly popular. Unlike their portable counterparts, these furnaces do not take up floor space in your RV, and are a more permanent RV heating solution - they can also act as an RV water heater.
On the downside, installing a furnace is considerably more expensive than purchasing a portable gas or electric heater. Unlike portable heaters, a furnace system can only be used in the RV - not your home, office, or tent.