Good morning
why3zx,
I have come up with a lot of potential uses for the drawer system other than it's original indented use, but using them as a pull out kitchen wasn't one of them. I think you are on to something here, and I'll certainly add that to my list.
Each drawer is rated to hold 200 lbs., so considering the size of the tanks you could fit in there you should be fine as far as a weight limit goes. A 5'5" bed length systems will accommodate 2 D-Boxes per drawer and a 6'6" will accommodate 3 D-Boxes per drawer. I have both of mine loaded up with tools, and off-road recovery gear, along with other heavy items, and they still roll in and out easily without sagging.
The angled sides of the drawers will cause you to loose some space if you use square or rectangular water tanks. However, if you want to maximize the use of that space, you may be able to use a couple of the
D-Boxes as tanks. The D-Box is 20.5" x 17.5" x 8", 1821 cubic inches, with a weight limit of 50 lbs. This means it will hold 7.88 gals. of water weighing 65.72 lbs., which is almost 16 lbs. over the 50 lb. weight limit, but I think you would be fine as long as the box was sitting in the drawer, as these are not flimsy boxes by any means. I'm pretty sure the 50 lb. weight limit refers to lifting the box with the handles. Each D-Box comes with two removable dividers, which could be left in place to act as baffles to help prevent sloshing while driving.
The DECKED storage system is made out of High-density polyethylene (HDPE), which is commonly a recycled product. The D-Boxes look to be made out of something different, possibly ABS plastic, but I don't find that information stated in the specifications. Personally, I wouldn't have a problem drinking water carried in the boxes, especially in an emergency, but I usually carry bottled water for that purpose. In the interest of liability I'll just leave it at that, and you can do your own research if you wish to go that route. Also, I would not recommend using these boxes for long term storage of water, since the irregular shapes inside would make effective cleaning somewhat more difficult.
Another option may be to use an approved bladder of some sort in the box. The box does have an EPDM Gasket around the edge of the lid to make it water tight. The manual instructs you to leave a small gap between the two ends of the gasket material when installing to function as a vacuum breaker when the lid is closed. Since you would probably be installing an air vent anyway, along with the other plumbing, I would close this gap up to prevent any leakage into the drawer itself.
The drawers have a small lip down the top of each side facing inward. I haven't tried it yet, but it looks to me like these could be used to hold a sliding 'table top' in place of the optional tray, as long as the sides of the drawer didn't bulge out. Each drawer comes with a cross brace bar, but that would just get in the way. From what I understand, the tray is no longer being offered just for this reason, and Decked is looking for other alternatives.
Another option IMO, would be to build your 'table' out of heavy gauge sheet aluminum with a right angled lip on each side forming a 'U' shaped profile. Then just sit it over the entire drawer. There is clearance down each side for it to slide with no problem.
Good luck with your project if you decide to go that route, and please post a link to your build on here if you do, so we can follow your ideas and progress!