I think Jeff is talking about excise tax, not sales tax. Several years ago the state started to collect excise tax when we get our plates/tags and it is collected by the DMV. Before that RV, boats etc were considered personal property and were supposed to be reported then taxed, but there was such a large number that slipped through without paying that they decided to change. Also the state instituted a property tax cap, so they were left with having to come up with other ways to find revenue :B
I was under the impression that they calculated the amount based on the fair market value. I know the tax on my rig has gone down every year since it was purchased, this year was a very nice surprise as the tax was about 1/2 of that last year.
On edit, I went to the BMV website and Jeff is correct that the class of vehicle is based on the Mfg Retail price, not the actual sales price. There is discounting every year to account for depreciation. I agree, that doesn't seem right, but that's the way they do it for cars also.