Mexico ... fishing in Winter

I know a very nice couple that truly qualifies as year round mega-
fishing bums. In past Winters they have moved their trailer way
down into Mexico along the Eastern edge and pursued sal****er fly
fishing.....somewhere

I don't seem to be able to locate them, at the moment, to ask them
directly
..... so I'll ask here.

Where in Mexico are good spots for a free-lance ( no lodge or
guides ), inexperienced at sal****er, angler to study on the map and
consider?

I'd be wanting to tow a trailer down ( although I might convince the
wife I "needed" a pop up camper and I have a 2008 Diesel that requires
ultra-low sulfur fuel (which I understand may be a real problem to
get south of the border??)