If this is for use while parked seasonally, it seems like an OK idea, but then again it would probably be just as easy to put the tank on the ground next to the RV.
If this is to do while traveling, check the DOT regulations on mounting or carrying LPG tanks. As I recall they basically had to be located within the framework of the vehicle: not hanging below the frame, nor above it on the roof, nor in front of the front bumper or behind the back one. The tanks also must be in a place well ventilated to the outside. The reasons for that regulation should be pretty obvious. I can't think offhand of any motorcycle carrying things I've seen on an RV that are not either a toy hauler (so not well ventilated) or that extend beyond the back bumper, so this is probably not going to be legal (or IMHO safe) for travel on the road.