I can only imagine that they got a bunch of big time investors and business loans to grow this out.

They simply do Fannie, Freddie, FHA, VA, and USDA loans.

A good tax accountant can save you thousands of dollars by leveraging entity selection and formation, tax deductions, cost segregations, bonus depreciation and tax planning.I recommend finding an accountant who specializes in business taxation, financial planning and tax planning.I would also recommend looking for a accountant willing to work with you throughout the year.

This means sizeable monthly payments, but if utilizing with Value Add you can pay off the loan without a pre payment penalty at any time.Hope this helps!

I know that interest on the acquisition loan can be included in the cost basis since it’s inventory.

Some banks and lenders will calculate your Heloc payment based on the Max limit even if you only have a small portion used due to preventive risk. 

I currently own and self-manage two STRs in Maine and should be closing on a small 'cabin campground' with 10 cabins nearby that will also be operated as STRs. 

I know a lender that does 15% down on investment loans.

If they have a good interest rate it might be worth looking to see if you could assume their loan to take advantage of the lower rate.  

Also, have you looked into DSCR loans for the refinacne portion?