The roads in Belize are fairly good to the Guatemalan border near San Ignacio. The Guatemalan roads to Tikal are good for all but 30 or 40 km. We would take our 34' 5th wheel to Tikal but have to get insurance before leaving US. You can get insurance at Florence but by then you are already 100 km into Guatemala and things may have already not gone well. We spent 12 weeks taking bus and shuttle bus through Guatemala and Honduras this last winter and loved it. We have a lot of friends who do go with their RV's there but they have truck campers.
The Federal Police have been invariably helpful as have been those in Yucatan. Tampico has had a bad reputation for a number of years due to a couple of police that hang out at one intersection. My wife just handed them the piece of paper the Mexican Government hands out requesting badge number etc. They conferred and just said "have a good day"
Reed and Elaine