A vacuum leak is certainly a culprit, though if it is intermittent, that seems to be not too likely. Once the engine hits operating temp, a vacuum leak is going to usually cause a consistent rough idle. I won't go along with the unlit propane around an engine that produces spark while running. Even though a build up of gas is unlikely, the consequences seem a bit drastic.
I would follow the FSM step by step in the troubleshooting section for a rough idle. I have done the same thing and it is time consuming, but also provides some thorough peace of mind about the ECU components. If you are not confortable with a multimeter, read the directions on them. They are easy to use and only take a little practice. Start with easy to get at stuff like the AFM and test it out. I think I mentioned earlier about the connectors and how important it is to unplug and replug them, not just look them over and call it good. This goes for the Ox sensor as well. Being under the truck, some people aren't willing to make the crawl or get dirty, but it has a connector like all the other sensors. Check the Ox wiring as well. It can droop and burn and cause faulty readings without tripping the Check Engine. And check your check engine codes, whether the light is on or not. Codes are stored, even intermittent ones. How long has it been since an emissions test? Any irregularities? How many miles on the catalytic converter? They do stop up, but very gradually. Well, just some thoughts.