Those equivalency numbers are pretty strange. They must be using "new math". As them how many amps or actual watts of output there are. They removed that information from their web site. I do not know if they are reposted it or not.
At 10 F I would need the DH50 and I'm in a 29 foot Class C with beefed up insulation.
I would insist that the installation used a fail over relay so that the propane furnace could be used as a back up.
I use a twin window fan to replace the cold air return for the furnace and I can heat electrically down to -37 C (-34 f). I get my cake and eat it too, because I can leave the propane furnace set a few degrees lower than the various electric heating units I use. As a bonus I can use propane and my electric heat at the same time. Last year that meant I got up to acceptable heat levels in less than one hour.
I would never retro fit a "cheap heat" system but if an RV came with one it would not be a deal breaker--so long as they would modify it to allow propane use on a fail over situation.