Is that a older Ford? From maybe the 80's -90's ? In that era the Fords had bad ignition switches and a bad plug on the back on the alternator. Both of which have been known to start a fire. I always changed out my ignition switches for a replacement and better switch right away and cut off and replaced the plug on the alternators myself.
Notice that it appears to be thoroughly burned out in the engine compartment. I wonder if that's where it started. The front of the RV is trashed more than the rear. If it started in the house I would expect the rear to burn down well before the engine compartment got involved.
Many times when a vehicle starts on fire it is an electrical fire. When a wire shorts out if a fuse doesn't blow the wire melts and fire occurs all along wire,many times this is in a harnass or under dash,etc .You have to unhook batteries before they can be even tried to be put out,most of the time by this time you cannot put them out.
I know once fires get going they go fast. But, it looks like he was driving down the road. I wonder why he couldn't put the fire out? But, I've never had a vehicle on fire so I have no idea of how fast it spreads or anything.