Plasti-mend

I've seen some use regular "Bondo" type plastic fillers with success. Just rough up the receiving surface really well, and use some screen for re-inforcement if you have big gaps. Did you strike the plumbing with something, or is the tank jumping around in the mounts? You might want to address that also. I put 1" nylon webbing over the top of mine in a couple of places to those T shaped tanks didn't hop up and down when empty. I love those big drains, but they don't have much flexibilty over rough terrain and the forces between 2 separate tanks.