Tammac Financial is a reputable company and has been around for a number of years. I did an extensive interview with the owner about six years ago for an article I wrote for Timesharing Today. The problem is that their rates are high (
starting at 10.99%) and the interest, unlike the interest on your Marriott loan, doesn't meet the IRS rules for tax deductibility.
You can get a lower rate (likely 7.99%) from
First Again, the only other lender I know of that makes timeshare loans. Again, the interest isn't deductible, so you'll need to earn at an after-tax rate of more than that on your savings to come out even.
For example, if you are in the 30% tax bracket, you would need to earn at an 11.4% rate on your savings to offset the non-deductible 7.99% interest expense.
A better deal, if your circumstances permit, is to get a home equity loan, as suggested above. The rate should be lower and the interest in almost all cases will be tax deductible.