As 6+ years FTing, we can also be added to the list of folks who use mobile RV techs almost exclusively (with the exception of the factory service center).
When you are FTing, leaving the rig somewhere for weeks on end, as I read on the forums so much, is simply not an option. And for many things, waiting weeks to get an appointment is also not feasible. In 6 plus years, we have only been out of the rig overnight for repair one time, and this was for a total of one night. (and that was our old Excel in which the company did a repair and put us up in a motel for the night so they could keep it in the bay).
As someone else mentioned, the techs we have experienced have done very good work, and could get to the problem within a day or two. They tend to get known around the parks, and I suspect the ones who don't know what they are doing don't last long as the RV parks are like very small towns--word spreads very quickly. I also think they tend to have many more years of service on the average than many of the people who revolve thru some of the service centers.