From what I read in your post "fifth wheel + boat + truck" makes me assume that you are pullin doubles (your boat behind your fifth wheel) if that's true then you need a class A. Unless you can somehow convince the police that it is for agricultural use. Good luck.
for all of you that are posting here is the rules I had to get it for pulling my train. dodge 3/4 ton 21 ft 5th wheel and 21 ft sanger. here is the law you can't go over 65 ft and you must have a non commercial class A. for more info go to DMV and ask for a book on RV towing with a 5th wheel I hope this helps. most of the CHP officers dont know the law we had to go to a weight station and ask for the info and than they told me what to ask for at the DMV