Another upvote, following some reading of the workarounds wyattic has been looking for, without much luck so far.

Although a couple years old, here is my guess how to get it working:
Remember to press the vehicle keybind "Turn on/off engine" within the 10sec window, it is not enough to just let go of the gas.
Perhaps it would be easier if you could simply "Rearm MK6 Mortar" via action menu on the mortar while ammo truck is nearby.