This is a well known issue among CA truck owners. It's also well known to the LE. Specifically, CHP. They will cite you for using a commercial weighted (pick up) truck as a non-commercial (auto) registered vehicle. I was reading the DMV website last night and it's pretty specific. I would look into a little deeper it myself before converting. The vehicle code says in effect "if you convert a commercial vehicle to a non-commercial reg., it has to have a PERMANENTLY attached camper shell with NO living or bath facilities and be under 10K GVW. According to them, a TC is considered cargo therefore, commercial.
A cop can easily determine your registration type by the last character on your plate. Commercial vehicles end with a number. Autos/passenger vehicles end with a letter.
I've read the stories of people getting stopped for having as little as a shovel handle sticking out of the back, which is considered cargo. The fine can be up to $2K according to the DMV website.