I did the R-134 conversion(evacuated and added 134 oil/refrig. w/r-12 comp.) 9yrs ago when my system needed recharging at $70/lb, I was never that pleased with it here in the CA sun. It would cool things down, but never made it anything more than liveable on 106+ day. I was never able to run teh a'c on low like I was able to do with the r-12. My comp was starting to make noise about 2yrs ago, so I looked for an alternative. I cane across a product called MaxiFrig, look here for a Maxi-Frig vs R-12. I am on my 3rd summer with the original same 2 cans I put in, no leaks, and no need to recharge. I don't have to run the a/c on hi, just to get cool enough to be tolerable, I have to turn it down to low because it will get too cold. I am not concerned with it being propane based, it isn't pure propane so it shouldn't be that volatile. Even if you ruptured an A/C line in an accident there isn't that much in the system to start a fire, more likely to start a fire from teh ruptured gas line and the exhaust manifold. The main concern would be a leak into the cab, though with any refrigerant a leak into the cab would be a concern, even more so with the 134 IMO.

Which ever way you go you need to determine why the current system is dry and fix it before adding any new refrigerant.