That is very serious, but fixable. You will need to take your TM to a welder, and have him close the gap and weld the crack closed, probably using a reinforcing plate across the crack. Or better yet, find a mobile welder who will come to your house and do it.
I doubt that installation of the WDH brackets caused the cracking, but at this point the cause doesn't matter. Several years ago we had a rash of frame cracks in this same area. What year is your TM?
One important thing for you to do before you go to the welder is get a good stiff wire brush and clean off every bit of rust and gunk on the entire tongue area. As I recall, multiple cracks were common, and some of them hid under the grunge.
Some discussion here
https://www.trailmanorowners.com/for...ighlight=crack
Bill