One possible benefit in WA state is if a trooper pulls you over and finds that you are not registered correctly (e.g., registration certificate indicates van or truck vice RV), you may be ticketed.
Another possible benefit in WA state is that you can carry open bottles of alcohol
inside the vehicle without getting ticketed.
Don't know why folks on-line seem to be indicating no insurance benefit, other than alternative methods for showing evidence of intent to be a low milage use vehicle for a discount.
BTW, again in WA state, I pay $117 for annual registration in my county. This is for an RV and it includes a voluntary state parks donation, plus all fees (you can find a fee breakdown on the DMV web page).