A couple of years ago I saw a Earth mover crush a 20lb tank in a garbage dump by mistake. The tank was buried in garbage and was there for an unknown length of time. It was crushed against a sharp edged large rock / small boulder. I was about 70 feet away - but I ducked and ran for cover - it was a big bang.
A number of years ago I saw one old 100lb tank in a cottage fire go up like a rocket ( the bottom probably was rusted) while the one beside it stayed intact.
Having seen these things happen I would not put any gas tank in a position where it vulable to impact during a high speed collision ( whether it is legal or not would not be part of my decision). Probably nothing would happen BUT it only takes one occurrence to cause at least a hell of lot of damage, or b) possibility someone horribly disfigured or dead.
Note: The earth mover operator just missed by inches losing his foot.
One other thought: If was legal or safe - would not the manufacturers install the propane tanks on the back bumpers? I have never seen such a setup on the road that was not obviously a handyman modification. Every RV I have seen in the past few years at the shows ( I have seen a lot - just bought my crossroads last October and i did a lot of research) had the propane tanks mounted in the forward part of the trailer / motor-home either on the tongue or in an enclosed cabinet.
My advice: If you need an extra tank - carry on the tongue or if you have a truck - upright in the truck box.
Edited by: Boomer
Boomer, the Better Half and our Yorkie - Ellie
2009 Chev 2500HD 6.0 L gas