Started by raising the bed and welding in 2 pieces of 1-1/2" x 1/8" wall square tube and so I could work under it safely. Also put in a piece of pipe on one corner, just in case.
First step on the repair was cutting out the twisted frame channels (6" horizontal and 3" vertical) so I could replace them with something heavier. 6" Angle grinder and my M12 Hackzall got it done with minimal damage to the bed and adjacent frame members.
The crack was very impressive, the factory welds were not. The 1" thick clevis was welded in with plain fillet welds, no bevel- and not welded at all on the back side. 1/8 stitch welds on the main mounting channel and cross members.

No wonder it tore loose; that changed the load on the hydraulic ram and bent the cylinder rod at the clevis since the load was no longer in line with the centerline of the piston rod. A few cycles more and this could have failed catastrophically.
I had some 2x3 x 1/4" wall rectangular tubing so I used that to replace the 3" channel. Put a crush sleeve in it and added another piece to carry the load path back to the next cross member. The clevis got beveled, welded in with 3/8 fillets and then I drilled a couple of 9/16 holes on the back side of the mounting channel ( same 6" channel, just patched the tear out and flipped it end for end) so I could burn in a couple of hot plug welds for additional strength.
Tested the fit-up, tweaked the cut ends of the existing 3" channel then added 1/4" fish plates for additional strength at the weld joint. The 3/8 bolts allowed me to fit everything up, I was working by myself and could have used an extra pair of hands.
Welded it all back together, put in about 4x the weld the factory used. Apparently they thought 1/8 skip welds were a good structural solution on a 10,000# dump trailer.
Also repaired the safety brace bar and built a simple locking mount for it. It had broken off at some point in the past and was duct taped to the axle...
Welds on the fenders were broken loose and the rear of the fenders were held on with a chunk of lamp cord, so I built new mounting brackets out of 1" square tube, flat bar and rebar. Beat a few dents out, welded up a few cracks, and added reinforcement plates at the center supports.
Calling this one done. Interestedly, the bent cylinder rod would not allow the bed to go all the way back down. It's up about a foot in the front ..He's taking it to the dealer for a new hydraulic cylinder on Monday.
Now, back to the big wrought iron project for the courtyard; masons finished up the stonework today. Would like to get everything off to powder coat this coming week.