For any that don't know, you can register your commercial vehicle for private/personal use only under VC 27603.
Not sure what you read but the code you cite doesn't state what you wrote.
V C Section 27603 Color Required for Former Schoolbus
Color Required for Former Schoolbus
27603. When a motor vehicle formerly used as a schoolbus is sold to any person and is used exclusively for purposes other than the transportation of pupils pursuant to Article 3 (commencing with Section 39830) of Chapter 5 of Part 23 of the Education Code, it shall be painted by the purchaser a color different than that prescribed by the Department of the California Highway Patrol for schoolbuses before it is operated on any street or highway other than to have the vehicle painted or moved to a place of storage.
The provisions of this section shall not apply where the ownership of a schoolbus is transferred to a nonprofit organization under a contractual arrangement under which the ownership is required to be retransferred to the original owner within 90 days of the date of the original transfer.
Amended Ch. 676, Stats. 1980. Effective January 1, 1981.
I have been in the transportation biz in California since 1984. I pay through the nose for everything especially insurance. If I KNOW
someone is operating illegally, I WILL RAT THEM OUT, if I have the time. Mainly because I know they are not carrying proper insurance. I only tell you this because I want you to know where I am coming from.
That being said, I will go off of what you said that you are not transporting for hire. I would write a letter to the Motor Carrier Dude and send it certified, registered or whatever they call it with a return receipt. Also find out who is his superior is and copy him. In the letter, ask exactly what he wants, what vehicle codes give him the authority to inspect a private vehicle and why he has chosen you for his butt sniffing expedition. Ask how he specifically singled you out and why. Tell him that you want all communication in writing. Tell him that you will not discuss this matter over the phone until answers your questions in writing. State the you will take this up the chain of command until you receive satisfaction and you feel that you are being singled out for harassment.
That is where I would start. Now if you were fibbing about the for hire thing, you are on your own. No more help from me.