Our house down there is the same way, with a metal roof, open air, then a concrete floor or roof above our 2nd floor living quarters. 

I know Air/VRBO are strongly pushing for hosts to offer non-cancellable bookings at a 10% discount. 

And also being next to a base Im thinking about short term renting, or Air BNB How did you finance this deal?

-Structural (foundation, wall, ceiling and floor)-Roof and Exterior(Roof Condition,Gutters and Downspouts, Siding, Windows and Doors)-Plumbing (Water Pressure,Leaks, Drainage, Pipes)-Electrical (Wiring,Outlets and Switches,Breaker Panel, Lighting Fixtures)-HVAC(Air Ducts, heating and cooling)-Water and Moisture Issues(Attic, Mold and Mildew)-Kitchen and Appliances-Bathrooms-Pest and Insect Problems-Safety Features

You can use bat insulation or spray foam, which has the added benefit of giving you an air-tight seal and also keeps the bugs out. 

Is it worth it to put in central heat and air?

Adding a 1/2 bath, new roof, central air or other typical improvements will increase the value of the property. 

I retired in 2008 with 21 years in the Air Force.13 sales in your first year is pretty impressive, particularly for someone on active duty.

The cold air often runs out in the summer and the heat doesn't turn on in the winter.

We have used actuals and projections from air dna to come up with rent.