Just about the last place I'd expect to find a food truck is in (actually near) Flemington in Hunterdon County. The truck is called Pudgy's Street Foods and it's usually parked on Route 523 near the ball parks at River Road. It's probably a mile from Route 31.
Before you get too excited however, Pudgy's has a pretty limited menu, namely hot dogs and fries. Not a hamburger to be found.
Pudgy's is even on Twitter so you can check where they are - they do move around a bit - and when they're open.