I agree with the weight thing and the difficulty in getting a ruptured cylinder to explode much less catch fire.
I was just wondering why you might want to move them to the back of the trailer. Just asking.
While I cannot say I have ever seen a cylinder mounted on the back by design, I have seen lots and lots of TCs with cylinders on a rear carrier. I think in the sixties and seventies that there was at least one TC manufacturer for every on hundred people in So Cal and most of them lacked sanitary tanks and propane tanks. Most were just outfitted with one five gallon cylinder or even smaller and owners added the carrier in the back for more storage and slipped a cylinder or two back there.
Just my observations.