For the back part of the 5er, I'm going to be using three MORryde X-Factor cross members for added Coroplast support. Unfortunately the way that Keystone ran the rear (galley) tank drain and the gas line, two of the three will need some modifications. Not a show stopper by any means but with the trailer 110 miles and in a CG from my home shop drill press and MIG, not easy. The unused spare hoist has two angle irons and acts as a support behind the wheels.
A note about using stainless hardware - most normal commercial grade is about as strong as a grade 2 bolt. This means that if rust binds it tight, you might just have as bad a problem as carbon steel.
I get all my specialty fasteners here:
http://www.mcmaster.com/# and below is a link to stainless as well. I used weather resistant coated steel on our now gone Titanium. They are also much less expensive then Fastenal and almost always have the item in stock plus overnight delivery with no tax
:
http://www.mcmaster.com/#catalog/122/3002/=138y9jw