Even if the weight is distributed over more than two framerails? I may be able to use two shorter framerails towards the center of the trailer, tied together to the outer rails. The leaves will be mounted on the outer rails, and probably at reinforced points.
I think the steel I'm thinking of is about 1/8" thick, but it's also not just regular box tubing, either. It has a few 90 degree bends in it on one side, so it "should" be stiffer than most.
I'll get pics of the steel I'll be using later today, and probably a quick sketch of the frame plans.