No special form is needed. Keep two pieces of documentation. One, a statement showing how much interest was paid to the lending institution on the loan. Two, a copy of the loan agreement showing the RV as collateral.
The bank has no reporting obligation to the IRS for mortgage interest paid on any loan that is not secured by real estate. If the loan to purchase the RV is not secured by the RV (i.e. used a credit card, a personal loan etc.), any interest will not be deductible. If you used a home equity loan to buy the RV the deduction will tie to your residence, not the RV as a second home. 2018 may have some changes from 2017 due to the tax code changes coming into effect.