Trading in an RV (or any vehicle) can add more complexity to the deal so you have to understand where all the numbers are coming from. Just because the current RV has a loan still on it doesn't mean that loan has to be "rolled" into the new loan as it is certainly possible that the Dealer will (for example) pay you 30K for your RV and you only owe 10k on it so that would be 20K reduction in the amount you need to pay (or finance) with the new purchase. Of course it could be the other way around and the amount required to pay off the existing loan is much more than the dealer wants to pay for your existing RV.
I will also add that at least here in Texas, a trade in reduces that sales taxes on what you purchase. Again for example only, if what you are purchasing cost 100K and you have a trade that the dealer will give you 50K for, then your resulting sales tax would be on the 50K remainder due instead of the full 100K selling price. ~CA