It isnt registered as a "housecar", "rv" or anything. It is registered as a "BU" which is cally gov speak for BUS.
I had the same thing happen -- and only found out about it this week, when it was time to renew and it came back as a CV (with those big fees), though it has noncommercial plates.
In this case, someone at DMV didn't put in all of the information on the inspection form, so the title type didn't change.
The difference was over $250, and I have sent an email to DMV to see if they owe me a refund for the amount I paid last year.