You likely will get a better / lower purchase price without a trade.
I was not financing, but last year I had three trade offers on my 2014 TT, purchasing a 2017 new unit. The place that gave me the best cash price also gave me the worst trade in price. They get their money one way or the other.
Not a lot of money involved in the deal but I bought outright at the best deal. Then listed my TT on CL at a great price and sold it in less than 2 weeks and got the amount of the best trade offer. Could have done better if I had waited till Spring. Figured I saved about 3000.00 on a pretty small deal, but still took a beating on the 2014 depreciation. You will usually lose $$ on a trade unless you have a trade in the dealer really wants / needs. Whether it is worth the hassle is another question.
On the financing question. If you pay down the principle after selling you RV yourself you will certainly reduce the interest you pay long term, although your payment will not normally be reduced because of the extra principle payment, unless the lender reworks the loan.