You would enter it in the same place you would if you had a 1098. You can deduct interest on a loan secured by your main home (where you ordinarily live most of the time), and a second home, even without a Form 1098. Just enter it into TurboTax as if you'd received the form.
A mobile home, RV, house trailer, or houseboat that has sleeping, cooking, and toilet facilities counts as a main or second home, as long as it meets certain requirements. Please see the TurboTax Help article Can I deduct my mortgage interest? for details.
To enter mortgage interest in TurboTax, search for 1098 and use the Jump to 1098 link to be taken to that section.