Just did some snooping around in the basement area with a flashlight, and it looks like the main structure is supported by the outer edges and there's one longitudinal structural piece that runs right about in the middle with holding tanks on either side. I think I'm going to try some 2x6's laid flat under each side and down the middle under that spar, plus ones at the front and rear width-wise. The floor of the basement is 1" or 1 1/4" ply, so I'm not terribly concerned about support under the whole thing. Only going to use 2x6's because of the longitudinal "ribbing" on my truck bed - I think a 2x4 would be prone to get in between them and not spread the load well.
