We have been traveling down thru Mexico for most of the past 15 winters. The only break in we have ever had was in the San Juan mountains in Colorado. A bad guy wouldn't even know there are holes in the roof. There are few traveling campers down there these days and you're more likely to mistaken for a bus when traveling than recognized as an RV. We have been to Belize and Guatemala, but always had a dog and they can be difficult and expensive to get into countries south of Mexico.
If you have a diesel, don't expect to find any ULSD or the DEF fluid south of Mexico. Mexico now claims to have ULSD, but you never know, our truck is a 2006, non ULSD.
For information find an old and out of date Church's Guide to Mexico, it and ioverlander.com are the best information out there. Dumping the black tank can get tough particularly south of Mexico consider swapping out the toilet for a cassette toilet which can be dumped at any gas station. For maps, there is a Guia Roji highway map of Mexico, probably available on Amazon. For GPS maps check out openstreet maps.
Most overland travelers stop at Overlander Oasis a small campground at Tule just east of Oaxaca. That is the best place for information further south and many of the people are from Europe but speak English.