Firstly, you are allowed to deduct any domestic property taxes you pay on personal (not for business) property as an itemized deduction. The property does not have to have a house on it. The deduction of property taxes is limited by the overall $10,000 cap on deducting state and local taxes.
Next, you can deduct "mortgage" interest on your main home and one second home. To count as mortgage interest, the loan must be registered (or "perfected") as a mortgage with the county records office. This means that if you stop payments, the loan can be foreclosed and you can lose the property. My guess is your loan would count as a mortgage on the land (check with the county if you don’t know), and it sounds like your main home (your land is part of your “main home” as long as you live on the land in your 5th wheel) so you can probably deduct the interest.
Then thirdly, if you take out a construction loan to build a house, you can count the interest on the construction loan as deductible mortgage interest, but only if the house is finished within 2 years of taking out the construction loan, and you must convert the construction loan to a mortgage when construction is finished. (If you end up not finishing in 2 years or not converting the construction loan to a mortgage, you have to go back and file amended returns to remove the interest deduction.)
