Never happened to ours, but, I have heard of it happening on several different trailer makes. The water tank is usually supported by the ends sitting on the trailer frame rails and that cross brace is more for positioning than support...that's what, IMHO, leads to their breaking. It is natural for the tank to flex up and down when towed down today's modern "smooth" roadways (
) with water in them since the primary support is on the ends of the tank, and the main trailer frame takes that weight, however, the flexing up and down will eventually lead to metal fatigue breakage towards the center of the stabilizer bar from that flexing.
Usually an easy enough fix; you could even replace the whole bar with a long piece of angle iron...,but, your plate should do the trick for a while at any rate