The IRS allows you to deduct interest on a mortgage that is secured by your first home, or your second home, but no more.
The definition of a home is, does it have sleeping, cooking and toileting facilities, then it qualifies. You travel trailer probably qualifies as long as you don't have two mortgages already on other properties.
In TurboTax Deluxe you do it as follows:
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Go to Federal Taxes >> Deductions and Credits
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Scroll Down to the Your Home section
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Find Mortgage Interest, Refinancing, and Insurance >> Select "Start" or "Update"
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Select the "Add a Lender" button
- This will lead you through a number of screens to enter your interest.