Yes, you can claim a deduction as a second home for the interest you pay on your RV. You would enter it the same as you would the interest for your primary residence even though you don't have a Form 1098. On the Mortgage deduction summary page, after you enter your mortgage for your primary residence click on the "Add a lender" box. Continue through the interview to claim the interest paid on your RV.
Note: You will need the Employer Identification Number (EIN) of the lender, otherwise you’ll have to print out and mail in your return with an attachment explaining why you can’t provide your lender’s info.