I don't have facts, opinions only. A rear end collision could be a major problem. However a well built cage designed to go up or down if struck could reduce the hazard. The present location of our tanks mid-way up on the passenger side rear, puts the tanks up higher, prolly as a safety measure. I have yet to see propane carried on the back of any rigs or trucks.
We have a back porch fitted to our TC. I don't carry the gas tank for our little portabote on it for this reason.
We're are struggling with propane lifting problem also. I'm considering a removable or swing out boom with a small block and tackle with a jam cleat, to hoist the bottle up to slip it in place.
I had a similar system to hoist a heavy dingy on deck by myself.
