I had considered full-length storage just like you're talking about.
When I drew it out, I saw two things. First, there's room UNDER the frame for boxes. Second, if I put the boxes under the front half of the frame only, that would help get me keep the weight centered ahead of the axle.
I built boxes around some frame members to get maximum width and length. The lids are between the first two frame members.
I have 3 folding chairs, a lantern stand, a tent, etc. in just one of these boxes. both boxes together total 3.5 cubic feet of storage space.
Only gripe is that the boxes took a long time to build and fit.
(The ancient Chevy truck rear axle has been replaced.)
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For build pix of Crocodile Tear, completed 10/26/06 -- Look at my album or new website <www.crocodiletear.com> (website has more info)