Wow Garry, that's a sweet setup you have there!
Eli and Seth: The drawers and cabinet are VERY rudimentary, not a whole lot going on there

The bottom of the cabinet is 5/8" plywood, the top of the cabinet is 3/4" plywood, the supports are 3/4" pine boards dadoed into the top and bottom, then I glued and screwed the whole works together. The drawers are 1/2" ply for the bottoms, and 3/4" pine for the sides. Right now they just ride on those plastic furniture glides that you can buy at the Crepot, I put slides on the cab-end of the drawers and the tailgate-end of the cabinet, so they are easy to replace when they wear-through. If the drawers get too heavy though another option is to install large, fixed casters in the cab-end of the drawers, recessed so that only approx 1/16-1/8" are exposed underneath so you don't sacrifice too much height. As far as the casters go, you should really get the polypropylene casters, because the cheaper plastic casters will square-off over time, and solid steel casters will wear a track right through the bottom of the cabinet!
Approximately 2/3 of my coworkers have similar drawers, ranging in height from just 3-4", all the way up to nearly 12" high drawers (mine are just over 8" tall to accomodate a Sys 3 or Sortainer). My second set of drawers were approximately 4" tall, just tall enough to fit two caulking tubes stacked, also tall enough to fit a framing gun, trim nailers, etc... (The drawers pictured about are my fourth iteration of the drawer concept)
As far as the drawer support goes, the drawers simply cantilever out onto the tailgate, and I made sure to triple my screw pattern for the last two feet of the cabinet closest to the tailgate. I also try to only have one drawer at full extension at any one time to avoid potentially ripping the cabinet apart.
The biggest snag with my current system is that when I'm hooked on to my Buggy (affectionate name I have bestowed upon my undersized trailer) I can't extend my drawers very far. To account for this, my new procedure is to either leave the trailer on-site, or if I'm towing it to simply unhook as soon as I pull onto the jobsite, which serves the complementary purpose of preventing others from parking so close that they prevent full extension of the drawers.
Lastly, the drawers serve an additional purpose as additional tool security. My Tundra came with a locking tailgate, so even if someone were to break into my topper, they would only have access to a relatively small percentage of my tools. And, even if they get past the tailgate, if I'm parked in a parking lot with someone parked behind me they can't extend the drawers very far anyways, so to access the tools closest to the cab they'd have to either have at least 8' clear behind the tailgate to extend the drawers, or they'd have to somehow cut the cabinet apart from above. I have all of my tools insured anyways, but I figure I don't want to make things too easy on the criminal element....