Although I have seen plenty of BBQ trucks that have no smokers, that all the meat is commissary cooked, there have been some that used Southern Prides, the gas option is a huge plus for many vendors. I am seeing a few of the Extreme Trailers of late, that have Backwoods, Spicewines or other cabinet type cookers showing up. These are very cool rigs and the cookers are hard to beat.
In my area, it has been a long time since I saw anyone with a food truck and a offset type or pull behind cooker, I suspect this is more about convenience and ease of operation than any legal requirement.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."