I put this under class A as I believe those driving Motorhomes ( especially longer heavier w/tag axles) would be paying much higher tolls than fifth wheels or travel trailers, and perhaps more likely ...
Some toll roads consider dual wheels as 2 axles. Some add up all the axles including your toad. So a tag axle MH would be 4 axles. Add the 2 on your toad and you are now a 6 axle truck.