I have found two keys according to my inventory, but there is a door that needs another key (right next to the door that opens behind the semi-trailer). I have searched everywhere many times and I am not finding any more keys.

The final key you need is located in a cavity in a brick pillar in the room full of crates. It's the room that has the sloped ramp leading up to it from the outside asphalt area.
The key is very easy to miss, It will open the metal door and lead you to Bubba.

Thanks a lot!

Totally thought it was a bug.