I suspect this price is based on the number of axles, not the vehicle type. As such they probably try to justify it on the basis of greater weight, and hence greater road wear. I know, RVs are lightly built and don't cause near as much wear as an 18 wheelers. But short of installing rolling weigh stations at the toll fixtures, what are they going to do?
Sometime ago someone complained about toll increases on a Maryland bridge. I found information on the original tolls. The ratio of toll to gas prices was substantially higher at the beginning than now.
Tollways are usually built with bonds, and the tolls pay those off. Even after the bonds are paid, tolls pay for maintenance and expansion. There's a clear institutional resistance to removing tolls, and switching the tollway over to tax support. Reading the wiki article on the Mass Pike, I see that tolls were removed for a while on the western section, as residents objected to funding the Big Dig.
