I am not sure how much is enough either, but the cost of adding reinforcement, the best we know how, is a trivial expense compared to another break. I'm going with the assumption here that the bracket I am getting is just like the one that already failed, until I know otherwise. That assumption would be raising the big red flag....
Guessing at the fail point (looks like it might be the arm to hub butt weld but things were pretty well torn up) from your old photos could help some. When you get the new in tact bracket and get a chance to study it a bit, it should become clearer where the weak points are. The welders that do a lot of repairs on car hauler trailers and trucks have a good sense about where things break and what can be done to strengthen things without adding a lot of weight and extra metal. You just want to be sure that any reinforcing metal structure not inhibit the motor mounting or airflow.
I do not know what is on the backside of the bolts that go to the fiberglass shroud attachement point. If that is just bolted through the fiberglass shroud and there were a couple of small washers there I would have four small metal plates made up to sandwich the fiberglass over a broader area, basically duplicate the rectangular piece that you have on the end of the new bracket, but I would have them grind and round the corners a bit to eliminate a stress point in the fiberglass. There may be some metal structure on the backside making that unnecessary but best to check it out and be sure.
If you do modify the new bracket with a welder it would be good to post some before and after shots, so that someone else can inspect the weak points that were reinforced at intervals on their coach and hopefully catch a problem before failure. They would also have a photo of what was done to your bracket to take to a welder for modification. Good luck with all this Larry, may the force be with you....
Later Ed