Under federal, "deductions & credits",  

You will enter it as a mortgage without a form 1098.  

1.  Click on the  "deductions & credits" tab at the top of the page

2   Scroll to "Your Home" section

3.  Click "Show more"

4.  Find "mortgage interest, refinance..."

5.  Click "start/revisit"

6.  Click "add another 1098"

7.  Enter lender name

8.  Indicate "seller financed and didn't receive 1098"

9.  Put in the bank information for your RV loan.