3200 pounds of payload requirement is barely a 3/4 ton gas powered truck from 2008+. 3900 pounds requires a SRW 1-ton, maybe dually.
MPG is all the same for 3/4 and 1-tons, bad. Diesel is less bad, but the trucks cost more up front. Figure 12-14mpg for gas powered late model HD trucks unloaded, slightly better highway and 10mpg with a TC. Diesel will be about 2-3mpg better under all conditions.
You can get away with a small travel trailer and stick with a half ton, but you won't save much on fuel. They tend to get 15-17mpg around town and at best 20mpg highway unloaded. Same 10mpg towing.
