You guessed right. For those, aside the non-commercial use, must meet the following: A camper top, with one or more side windows completely encloses the truck bed and having seats, seat fittings or camping equipment installed in the truck bed.
Good. Where can i see differences between classes?
You can refer to DMV Regulations and specifically on Part 106 of such rules.
I'll be able to find what kind of modification made possible that my vehicle be one or another class?