Here is a good article about
Doc Fees relating to cars.
I've purchased five RVs and numerous cars, all tried to add on extra fees, I've walked from several dealerships that wouldn't negotiate. It's pure profit if you pay them. Some States do have established Doc fees, most don't though. $350 is outrageous, $35-100 I might agree with if I was getting a great deal. Out of State fee? Are they meeting you some place with the RV for delivery? If not I'd refuse to pay that one period, at least without a reasonable explanation.
When I bought my Jeep the owner of the lot and his sales guy all shook hands on the value of my trade and the sales price of the car, I even stated final price before I shook on it. Singing the paperwork they added a Doc fee, I reminded him of our agreement, he tried to negotiate a bit and I reminded him I had his word of final price. He agreed and I drove off happy. They didn't want to lose the sale over a few bucks.