For backup in case a heater should ever quit working and make miserable or ruin an otherwise great campout, I recommend going with two Wave3s (good) or two Wave6s (best).
Using two heaters you can place one heater at one end of the RV and the other one at the other end of the RV. Each heater can be set to a different temperature for uniform comfort throughout the coach area.
With two Wave6s and both operating, of course you could handle unexpected very low temperatues. In medium low temperatures if one should fail, the working one can get you by.
FWIW, we're in a motorhome instead of a towable and as such we have three independant ways of keeping the coach warm in case heating systems should ever fail during cold weather camping situations.
