There are a few ways to handle this issue, as propane is a limiter for me when boondocking, especially in the summer time and I'm running the onboard genset to handle the heat. Here are a few ideas I'm looking at:
1: A hitch mounted cargo rack. Some campers have the ability to have a tow hitch mounted that can handle a couple hundred pounds. From there, hit etrailer, get a
propane rack that would bolt onto the cargo rack. Then, drop your bottle on, screw it down, and call it done. Another trick is to get a cargo rack like
this which, if the propane bottle is short enough, will ensure it goes nowhere, but is well vented.
2: A mounting for the front of the vehicle. I've seen cargo racks on the front of pickups before... but don't put too much there, as it might affect radiator performance. One can then use a cargo rack there.
3: If neither works and one is a good carpenter (I personally don't have the good skills, nor would I want to drill holes in the side), one could affix a
rack to the rear of the TC. I have never tried this, but just throwing it out there.
4: There is always a trailer option and stashing propane on a
rack on the trailer's tongue.
5: As a last resort, store it in the truck or truck camper, well ventilated. When I see a propane seller sending bottles to fill up the cages, they don't use anything fancy for a cargo truck... just an enclosed semi trailer with no ventilation whatsoever. So far, I've not read about any of those exploding.
I may be far off base with these, but it is a place to start.