Same here in Oregon, once you are over 9,999 lbs it is a different registration ('T' plate) and much more for registration, as it goes from <$50 per year when under 10,000lbs, to $450 per year for 10,000-12,000 lbs and on up from there, OUCH! I'm guessing the OP is going to be a 'motorhome', so that gets around the 'truck' registration, but still more per year that an auto/light truck (RV fees are based on length here) and any 'detachable' camper has to stay on the truck, no mix-n-match.
Bizarre how much state Reg fees varies!
Here in CA, which is known to squeeze ever last penny out of us, it was a surprisingly HUGE drop in Reg fees when i went from a truck to a "RV"
Almost $600 a year, went down to $120 a year after converting it.
This was something i hadn't know about until i posted my truck/camper for sale and everyone kept asking me if it was registered as an RV. I was confused...
But apparently if you are financing something like this, you can get a 20 year RV loan at low interest. Not my thing, but for each is own.
Looked into it, took me literally 10 mins at AAA, and it was even free!
All states vary, but as the clerk explained it to me during inspection, it needed to have 110v power, a toilet, and something to sleep on.
I've heard many state the fines are astronomical if you get caught driving without the camper on it after you convert the reg, but don't know anyone first hand thats actually gotten the ticket, so who knows..