I know of no law that states that you can't, but...
Several years ago, we were heading down I-10 toward Mobile, AL pulling our TT behind us and a truck pulls up beside of us and motions us to stop. They go on and another vehicle comes up and does the same. We figured we had a problem and DH pulls over and goes to the rear. Our propane tank had come loose when the bungee cord holding it broke and it was dangling behind the camper, bouncing on the road. It was behind held onto the rack by a piece of hose that was entangled in the top of the tank. DH cut the hose loose and slung the tank as far as he could away from the road. It never did explode or anything, but it scared us and I'm sure everyone behind us.
2017 Ford F350 Crew Cab 6.7L 4x4 DRW