Talk the salesman into listing it as a motorhome, you will avoid any DMV/CHP inspection hassles. On my previous truck I just told my salesman the camper was going on as soon as it got home, and offered to swing by with it on, if they wanted to verfiy, which they did not. They should be accommodating. You don't say what type back you have (box, flatbed, chassis/cab, etc), but the chassis/cab alone would be tough to pull off.
Update:
from
DMV form REG 256A (
http://www.dmv.ca.gov/forms/reg/reg256a.pdf)
Human Habitation is living space which includes, but is not limited to: closets, cabinets, kitchen units or fixtures, and bath or toilet rooms.
from
California Health and Safety Code 18010.
"Recreational vehicle" means:
(a) A motor home, travel trailer, truck camper, or camping trailer, with or without motive power, designed for human habitation for recreational, emergency, or other occupancy, that meets all of the following criteria:
(1) It contains less than 320 square feet of internal living room area, excluding builtin equipment, including, but not limited to, wardrobe, closets, cabinets, kitchen units or fixtures, and bath or toilet rooms.
(2) It contains 400 square feet or less of gross area measured at maximum horizontal projections.
(3) It is built on a single chassis.
(4) It is either self-propelled, truck-mounted, or permanently towable on the highways without a permit.
Calif. vehicle code 260.
(a) A "commercial vehicle" is a motor vehicle of a type required to be registered under this code used or maintained for the transportation of persons for hire, compensation, or profit or designed, used, or maintained primarily for the transportation of property.