hi KVVMPU, and welcome to teh site. Though it does say 7 posts, so this may be late or maybe I've already said hello. I cna never remember.
I suspect the use tax is going to be based on the price on the Bill of Sale. You might want to ask them to write a seperate Bill of sale for the trailer and the boat. In many states (Note these regulations vary by state) you might have a hard time registering the trailer without a Bill of Sale. I would certainly check with the DMV first. If you can, try to get the seller to split the boat, trailer and misc items onto three Bill of Sales, that way you reduce the use tax on the boat, and are more easily able to both register and sell the trailer at a later date. Combining boat, toys and trailer onto one bill is messy, in my opinion.