Check out this company:
80/20® Inc. - The Industrial Erector Set®
The pro: infinite possibilities, no welding, very strong cross section (i.e. resist bending, torsion, etc)
The con: cost
(not affiliated, just posting possibilities)
They are self-proclaimed as the "industrial erector set" and I believe them. We've used some of their material to construct low volume/prototype coating/dipping stations where I work.
Ideally, this is what I would use if & when I build my own drawer system. By being an 'erector set', it can be somewhat re-configured. In my mind, the top of my 'platform' would be multiple pieces, each hinged to form a top-accessible hatch (i.e. access to drawer contents from within the truck). With all 'hatches' closed, I have a sleeping platform.
Wow, talk about one stop shopping! That source looks like just what you need. My only problem is I have absolutely no experience in designing and building such a thing.

Now if someone had detailed plans and a parts list...

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