Looks like I was wrong with my interpretation of the CA motor vehicle statutes. Apparently, it's not enough to have a permanent camper shell on a pickup to qualify for the weight
fee exemption. According to their definition of a camper it has to be habitable with a sink, bed, etc. So, it turns out that all pickups are considered commercial vehicles according to the DMV. The additional fees are based on the weight
of the vehicle. Goofy because even though a Ford Excursion weighs considerably more than a Toyota Tacoma, the Tacoma is considered a commercial vehicle and subject to additional weight fees while the Excursion is not.