When I departed McCarran all of the buildings were showing
This means your problem doesn't have anything to do with anything that it's being discussed here, which is the lack of Addon Manager and/or Couatl right from the start. If you saw buildings when FSX started, you surely also had both menus and programs working, otherwise you wouldn't see any buildings, right from the start.
after I flew to Reno and then back I noticed no buildings just the jetways appeared. I noticed the same problem on Sunday when I flew LAX-LAS (buildings appeard in FSDT-LAX) when I landed at McCarran no buildings just jetways.
Next time you start a flight at an FSDT airport and you HAD buildings at start, but they disappeared after a long flight, check if you still have both the Addon Manager menu AND the Couatl menus in the Addons menu. And, check the following file:
If you have that file, post its content here.
No Addon Manager or Couatl (or both) at start it's one thing, but one of them missing only after a long flight is entirely different, doesn't have anything to do with antivirus, etc, and requires entirely different troubleshooting steps so, it's important to be sure which case it's yours.