There are many tax accountants on Bigger Pockets that can help you with your situation. 

Looking for various tax strategies to reduce the reality that a large RV or boat is a grossly depreciating assetI would not have a loan, so claiming it as a primary or secondary residence and claiming the mortgage deduction is not applicable.I sold my primary residence and I have several rental properties which I use as I'm fixing them up.What I'm pondering is living aboard and working aboard developing a new product, while traveling.  

If your friends have a rental they should hire a tax professional. 

Quote from @Joshua Tucker: What specific tax benefits can high net worth individuals receive from building real estate on tribal land?

With over a decade of real estate investing experience and over 100 flips, I now provide private money & hard money loans. For my own portfolio I'm focusing on commercial deals and coaching fellow RE investors. I ...

The first year my property taxes were $1,619.

Fannie/Freddie (Conventional) loans aren't just about rate and transfer taxes. 

The only taxes I remit directly now are the tiny town tax they implemented of 1%.I just take my net income spreadsheet and calculate the 1% and give them a check.

if it is unsecured loan that is the consequence, you lose they entire thing. as far as tax write off, sorry but none.

Taxes and requirements should be much lower for a LTR