Mortgage interest may be deducted on a timeshare loan. The interest is deductible only if the loan is secured by the timeshare as a mortgage.
Per IRS Publication 936 Home Mortgage Interest Deduction, page 4:
Time-sharing arrangements. You can treat a home you own under a time-sharing plan as a qualified home if it meets all the requirements. A time-sharing plan is an arrangement between two or more people that limits each person's interest in the home or right to use it to a certain part of the year.
Rental of time-share. If you rent out your time-share, it qualifies as a second home only if you also use it as a home during the year. See Second home rented out, earlier, for the use requirement. To know whether you meet that requirement, count your days of use and rental of the home only during the time you have a right to use it or to receive any benefits from the rental of it.
Second home rented out. . If you have a second home and rent it out part of the year, you must also use it as a home during the year for it to be a qualified home. You must use this home more than 14 days or more than 10% of the number of days during the year that the home is rented at a fair rental, whichever is longer. If you don't use the home long enough, it is considered rental property and not a second home. For information on residential rental property, see Pub. 527.