Because I have not read your original loan contract or the new document Wells Fargo wants you to sign, I can only speculate as to the contents of either. You should accept the following observations accordingly.
I would be surprised if Wells Fargo did not already have terms in your original loan contract stating that you are liable for any deficiency balance upon repossession. In fact, I would be more than surprised if it did not. My guess -- note my word choice -- is that the new document repeats what is already in your contract and adds that by surrendering the RV you are giving up your right to possess it.
I see no reason why you cannot write your own drop-off document and ask the branch manager to sign it. I would be surprised if the branch manager has the authorization to do so, and as a result you may leave the office without signing each other's forms.
To learn more about this subject, see Voluntary Repossession.
I hope this information helps you Find. Learn & Save.