This is not going to be an easy fix. You might take it to a frame shop and get it pulled back straight, but that won't prevent it from happening again.
If I were going to straighten it with what I have to work with...
I would remove the fenders and 3 more of the cross members, grind away all remaining welds from the fenders and cross members. Then make a series of cuts about half way down the box tubing with a plasma torch, sawzall, or grinder (not torched) about every 12 to 18 inches or so, on each side of the bend, probably about 8 cuts per side, v the cuts with a grinder. Jack up the trailer from the rear and weld the top of the cuts. Don't tack it, weld it straight across, weld it hot. This should draw the frame back to straight. Continue to weld the sides of the cuts and grind smooth. Then plate the inside or outside, or both, with 3/8 x 8" flat plates. This will pull everything straight if need be.
Lots of work, but a better trailer when finished.