I think part of my point is there a certainly plenty of qualified diesel mechanics in Mexico as there are lots of diesel powered vehicles.
Can't find a part? I know there are probably more than a few ways to get parts to Mexico with one being . . . http://www.merkalink.com/English/como.asp . . . which is either part of or hooked up with Estafeta. Not saying either of these is the best but ways of getting things done do exist.
Many things we do and/or need to do while in Mexico may not be as easy as they may be in the US but it is possible to get things done. I am not sure that it matters whether the vehicle is gas or diesel - it could be difficult to find some parts in Mexico. But some things can be found.
We have a Freightliner chassis with a Cummins engine and have had mechanical problems while in Mexico. By searching and asking and maybe more than a little leg work I have been able to find parts or find someone who can make parts. I have had both hydraulic and air lines made up at places that sell hoses and fittings for example. And I do carry some parts and tools with me.
Specialty items may require more perseverance. Figuring out what is needed and finding someone who will ship from the US can be done.