I'm a bit confused as some lenders incorporate prime, others use 1-year treasury and others are offering a fixed rate...This will be my first HELOC.

There are many reasons to insure your property cash flows as you are taking on a much bigger risk than, say, 30-year treasuries, so why risk little or no return until hopefully the tenants pay down the mortgage for you?

Treasury Department. 

Treasury that would allow federally chartered banks to move onto their turf."

@Natalie Kolodij and @Jake Hottenrott, the issue here is that if a property's average period of customer use is 7 days or less (as many if not most Airbnbs are though I'm not sure about @Sara Erickson's case), it's not considered a rental activity under Reg §1.469-1T(e)(3)(ii)(A) and therefore would not be depreciated under 27.5 years, which is reserved for residential rental property under §168.That being said, the scope of Reg §1.469-1T(e)(3)(ii) limits the scope of Reg §1.469-1T(e)(3)(ii)(A) to the 7-day-or-less rule to Reg §1.469-1T(e)(3), not §168 and not the entire IRC.And of course the regs for the 7-day-or-less rule are temporary regs issued in 2002, and the shelf life on temporary regs is 3 years.But nevertheless these regs probably give some insight into Treasury's thought process here.Speaking of which...Airbnb wasn't a thing in 2002, and Treasury's presumption back then may have been that the "rental" of a property with an average period of customer use of 7 days or less would likely have been bundled with some significant services, thus making the entire activity a trade or business rather than a rental.But things are different now, though obviously some services are provided with the typical Airbnb (e.g., the provision of clean sheets, television, complimentary snacks, etc.) that aren't provided in the typical rental...What do I do?  

It would definitely be much easier (and probably about the same on total cost) to just cashout refi what funds you might need from you STRs and then park then cash in MMAs or 30 day treasuries until you need them.

Invest in a treasury.

Out of state investors and many international can get sucked into a proforma and think they're basically buying a treasury bond with a guaranteed yield.  

When I asked at the city treasury dept if I can pay off delinquent taxes (3 weeks before auction date) I was told it is too late and the house will be auctioned  off. 1.

Bank CDs or treasuries are the seminal passive investment.