Read here (http://www.irv2.com/forums/f44/elect...eat-97637.html
) for a thread by a guy who actually installed one. So far, he has used it only in temperatures in the low 50s but said it worked great in those conditions. He says he will report again when he has used it in colder weather.
Even if the BTU output is less than the propane furnace, running it for longer periods of time might be able to still keep the RV warm enough. If the temps drop too low for the resistance elements, the propane furnace would still work.
My personal opinion is if one has heat pumps, they would be more efficient (thus cheaper to use) than resistance heating although that advantage would lessen as temperatures crept lower. Below a certain point, resistance heating would be cheaper and/or more effective than heat pumps. Of course, LP would probably be less expensive to use than heat pumps or resistance heating unless one is only paying a flat rate for the electricity instead of being metered.
There are other factors to consider besides operational costs, such as not having to refill the propane tank(s) frequently in colder weather and the ability to leave the heat on instead of winterizing with glycol to protect the plumbing from freezing when gone for extended periods of time without having to worry about running out of propane.