just some suggestions off the top of my head to make it a "do all" cooker(obviously use it horizontal as it is in your truck bed to maximize cooking area)
1. closeable chimney stacks on top and vents on bottom at each end. will allow you to adjust air/smoke flow and temp.
2. pull out cooking grate is a must.
3. removable-full length charcoal grate in sections, not one piece.(no fire box).
4. if you do #3, you can still smoke by having the fire on one side and meat on the other. on fire side, close chimney vent and open bottom vent. do the opposite for the meat side.
5.multiple thermometers. # and placement dependent on how detailed you want to be.
6. modify the trailer so that the tank is supported at least at all 4 corners.
Weber-18.5" Kettle, 2 Performers, 18.5" WSM, NYG Smokey Joe