So we stripped the chassis down to just the welded components and the rear axle, easier to move around on one axle since weight is not an issue at the moment. We are ordering new axles soon since we determined that bad axles was definitely the cause of the excessive crown with a slight buckle between the axles.
At first glance there seemed to be no bends or anything serious that would cause the crown, but on further inspection after we media blasted the entire chassis did show a slight kink in the rail stiffener that is welded to the C-channel frame rails. The stiffener is a large thick plate welded to the side of the C-channel over the axles and there was a buckle between the axles on the curb side.
Our fix was to tow it down to my friends welding shop in Redondo Beach and jack up both ends of the frame rail and pull down right at the kink. While it was stressed we clamped a piece of 2"x3" heavy angle iron to the inside of the rail stiffener between the 2 axle mounts. As we welded the angle we kept re-clamping and sucking in the kink as we welded towards the buckled spot. By the time we finished you could not really see any more kink in the top rail.
After working out the main issue we took 1/8" x 5" plate and cut pieces to fit all the way from front to rear in between the cross braces or truss pieces or whatever they are called. We then stich welded them with about 1.5-2" welds every 6-8" staggered top and bottom. The channel iron spreader bar that was welded at the rear of the chassis was rotted so we replaced that with square tube steel.
The end result is a chassis that looks dead straight now and while it still has some flex is MUCH stiffer than stock. We are very happy with the end result and can move forward with much more confidence in the foundation of the trailer.
Thanks for all of your input...more questions to come.
BTW, I bought a gallon of POR15 to coat the chassis to convert any spec of rust the blasting did not knock off and help prevent future issues. We will use a dauber style brush on a heavy wire to coat inside the now boxed frame rails since there is still some access every 24" or so. I will then top coat paint the exposed areas of the frame a nice silver later to UV protect it.