In my experience a retail bank (especially the major banking chains) don't give a better interest rate than a car or RV dealer. Credit unions may be able to offer lower interest rate but that comes with larger down payment requirements and shorter loan terms than dealer financing can offer.
Much we don't know about the OP's situation financially though he did acknowledge some significant factors about income and employment. The dealer offer on the RV he was offered might be the best he can get. Since it's a simple interest loan offer, he can always accelerate the payments if he can afford to and pay the loan early without penalty. Our last RV purchase was financed for 15 years, we paid it off in 7.