Has anyone done this? Placed a flat water tank on the storage platform in the crew cab when you may need to have extra water. Use a small pump to transfer to the TC tank when needed. If it was flat...
You might consider one of the flexible water bladders / bags like they used on my sailboat. Much lighter than a poly tank, easier to move in and out of your truck when empty. Attachment points at the corners for anchoring where you want it. Comes with 1-1/2" fill inlet connection and 5/8" outlet.
Could build a light frame if you really want to store things on top of it.