AHHHHH more puzzle pieces...
Your tube frame has very little resistance agasint tube crush. WOW...
This is a tell tail pic. and the others are not great either.

Your plate idea is for sure in need now.
The bent DC chain keeper plate, odds are high, that was bent when the DC frame plate was bent in. I have seen that chain keeper bend in that area before from guys trying to snap up under full load rather than jacking up the truck and camper hitched. It really does not take much force to bend the chain plate where it is key hole attached to the DC arm
The new Reese gusseted snap up may work on a channel iron frame unbolted or on a heavier tube frame that will not crush so fast. In your case, I agree, fix the frame and then bolt on the snap ups with no play to allow the bracket to slide down. You will need to deal with the DC frame plate too. Once the fix is in, if you have access to a welder buddy, some stitch welds on the plates to the frame may not be a bad idea. I do not see needing a lot of them.
Good luck and hope all this typing helps.
John