While we haven't driven a MH through Mexico, we have been flying to Cancun for the last dozen years and then driving a rental car 240 miles south to Xcalak. The place we stay at does their shopping at Chetumal. All the roads from Cancun to Chetumal (and likely the Belize border) are good paved roads. No banditos, but an occasional check points that rarely stop a tourist. Never been stopped by the federales or had a bribe sought. You do need to be alert at the gas stations as the pump operators try to pull some fast ones on occasion.
If I were younger and had a newer MH, I would consider it. However, we see no reason to go beyond the Yucatan Peninsula. By the way, there is an RV park at Paamul between Cancun and Tulum.