Unless things have changed, there are two requirements that I know of for the interest to be deductible.
1. The mortgage/interest must be secured by the real property itself. As mentioned, a personal loan doesn't work.
2. You are allowed only one (that is a single) 2nd property. This includes any fully equipped RV's or Boats. (Fully equipped means they have cooking, sleeping, and bath facilities - or in other words are fully self-contained). Thus you could not claim interest paid on loans for a) your house, b) an RV, and C) Disney, all at the same time.
That's what we had in our case, but that was many years ago. As always, your case may be different. So check with your accountant (or call the IRS) to be sure.