You can deduct interest on a mortgage for your main home and one second home. To be counted as a "mortgage" for tax purposes, an RV loan has to be secured by the RV so that it could be repossessed if you stopped making payments. An unsecured loan (like a credit card or personal loan) to buy an RV would not count. To deduct mortgage interest without a 1098, enter the name of the lender and check the box for "I didn't get a 1098 for this loan".