Being from SC I have seen, and competed in a "few" BBQ events. We actually won one event last spring. :D What a great time. One great "addtion" to a rig that I just saw was a small "computer type" fan added to the smoke box. It pushed the smoke across the chamber quite well. Have a designated place added on the trailer for your wood, to carry to and from the events. Consider the extruded wire box with a lid, chunks of wood tumbleing down the road when they get bounced out can get ya in trouble. You should also add a place for water and catch basin, health dept. likes to see that. Are you going to be using Stainless steel grids for a cooking surface.. if not mebbe you should consider it, easier to keep clean when you get home, pressure washer, or stop by the car wash. Also add a place for coolers to carry product, NOT the small coolers but the 128qt type, Marine type coolers definately hold ice, product better. Also add a couple of thermometers to the top of the unit, afterall tempeprature control is essential when cooking "Q" Also a tongue jack to jack the unit off the truck, and a couple of supports welded on the corners to ensure stability and the unit being level. Also TWO burners for pots of beans, brunswick stew, etc.
Plan for the unit to be totally self contained so you can set up anywhere and cook.
I have lots more. If you private message me I can direct you to some sites to see pictures of other rigs, trailers, and portable pits. Best of luck
As far as the Kitchen goes, it is a long, long day that is never really over, you just go home at some point