You are allowed to claim mortgage interest on your primary home and a second home. A RV, motorhome, or boat will be considered a qualified residence if it includes a sleeping space, a toilet, and cooking facilities, and you are not already claiming two homes.
After entering your mortgage interest paid for your primary home, you will arrive at the "Home Loan Deduction Summary" page.
Under the word "Total", you will see an +Add a Lender link. Click and enter loan interest paid information on your RV.