Thought I would provide an update....
The Plasti-Mend kit came in. Today I started grinding away whatever they used to repair it. It's not silicone like I thought. It's some sort of hard epoxy/filler. The more I ground it away, the bigger the crack got. It's huge. It goes up to where I can probably reach it with a brush for the resin and repair mesh, but it's going to be tough to clean up in there. The crack is wide so I ordered some ABS plastic as well. I think I can fix it - I really don't want to have to replace the tank.