I have seen rear mounted Propane tanks, Epically on Truck campers.
however most TTs and 5ers the rear bumper is a total joke, basically foil strong and the weight of a propane tank will very likely pretzle it a few bumps in the road away.. If rear mounted it would need to be mounted to the body and to a stud.
I would not do it.
Plus as someone said.. If you get rear ended, not a good thing,, And people LOVE to tailgate.. I checked the idiot following me the other day (Wed or Thu forget which) at leass than 1 second behing my door post (I was in a car) proper distance is the time it takes me to travel 3 Seconds Or more) and that's from the Rear bumper So he was like 1/4 the proper distance.. had I stopped fast he'd have been in 1: My back seat and 2: a very uncomfortable position in court.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times