There is a very wide range of choices for heaters available for RV's, from high efficiency stainless boilers
to common water heaters using fuels from propane, electric and oil.
Solar is not practical because of the lack of storage.
Diesel is best for heating because RV's are very inefficient and require a lot of heat and propane is limited.
The water heater used in most RV's can not be used for heating.