There are some really fantastic and long term Investors here who have made it through Crisis after Crisis and we know that is a Myth.The 2nd Myth is that High Valued Properties will suddenly crash.

My sellers had low equity and had sudden job transfers and weren't planning to buy again soon. 

A ceiling does not suddenly start bowing after 7 years, if you see the drywall crack, it's probably been doing this for 7 years.

The long term investment offers more investor friendly options like faster appreciation in equity, easier to take cash out and finance.You also do not have to worry about having to all of a sudden lose cash flow due to an increased lot fee or restriction in any way to you or your renter due to park rules/regulations.

However, as we approach the closing date, they've suddenly requested a higher down payment of 40%, citing a "declining market for short-term rentals."

I just recently closed on a rental property in LA where the zestimate was 660K (it sold for 640K last year in June) I managed to get the purchase price of 516K which I'm excited about as I'll have over 100K in equity however once we closed and it went into pending the zestimate on Zillow suddenly dropped to 535K!

I have been doing deal analysis for awhile now but this one property suddenly caught my attention.

Yes, as a military Officer I can still be "fired," but unless I suddenly become a 'bad' Marine, that is unlikely to happen. 

Btw, I experience this every day as an investor -- people hear that I "flip houses" and suddenly I'm scum of the earth.

Sounds like you had a great time :D We won't get too concerned unless we hear you had to make a sudden trip down to the island, Mon!