Is mortgage interest on an RV or mobile home tax deductible?

Yes. You're allowed to deduct the interest on a loan secured by your main home (where you ordinarily live most of the time) and a second home.

A mobile home, RV, house trailer, or houseboat that has sleeping, cooking, and toilet facilities counts as a main or second home, and as long as it meets all the other requirements for deducting mortgage interest, you can claim the interest like an immovable home.