Boondocking in Mexico?

We have a motorhome they sometimes call a bus (autobus)with two axles and dual wheels in the rear. We tow a car. Most times they rate us as a two axle bus and a car - add the two tolls together. But then sometimes they charge us like a 4 axle truck. We use the online toll calculator but it can be a bit out of date at times so we also watch what the board says coming into the booth to get an idea of what the toll should be. To our knowledge we have never had a toll attendant short change us. Calculating/computing tolls accurately can be an impossible task. We get a pretty good idea from the online calculator so we have a close ballpark idea of the toll. Each time we go through a toll we write down the amount in our ongoing toll maps (from the Bell's downloadable route guides) and have found they pretty much go up a bit every year. Many toll roads are privately owned by corporations on some sort of deal with the Gov't. They must follow certain rules. But the corp. builds the roads and maintains them. It get complicated. We do try to use routes that avoid tolls and in recent years we find many other roads have been greatly improved and much more usable. Specific road info can be learned by asking questions here or by experience.