I've not done the conversion on a Monty, but I have done it on many road tractors. A system converted to R-134 does not cool as well as the R-12, but the difference is negligible. And in the Monty you won't notice it much. Also, before everyone gets up in arms, the systems that are R-134 from the factory cool just as well as R-12. The conversions are the only time I ever encountered a difference. As far as reliablity you should not run into any problems. And of course the biggest factor is the cost and ease of getting the 134. The only way you can get the 12 is take it to a shop or be certified on A/C and then it is more expensive. I hope this helps.