There is one that is ALWAYS at the Fort. It is the FW/Disney food truck that's at the dining page.
The linked thread indicates that there are outside (non-Disney) food trucks that come in and they usually park around the paved area between Trails End/Settlement TP/boat dock on the mid-week days when TE is closed. Reports have been that the number of outside food trucks can vary week to week. Although this was before HDDR restarted (don't know if that matters).
Of course, P&J's is available also.
Bama Ed