Holding Tank Crack

It seems every trip has a new problem and our latest was no exception. I have a 10" crack in the bottom center of the ridge detent in our holding tank. There was no apparent leak or problem on our first two trips this summer so I don't suspect it is a freeze problem. However, it is at the greatest flex point of the tank and I suspect with no external support the flexing of the tank caused the crack. Any thoughts or others who have had a problem?

In any case, preliminary estimate is $700 for a new tank (installed). I haven't found out yet how much is labor and how much for the tank and will be looking at doing it myself - I haven't been impressed with the workmanship I have seen on a couple previous repairs at the dealership that I had to then redo. Anybody changed out the tank? I have tried calling TM service but got voice mail and last time it took 3 days for a return call so this forum has proved far more useful.

My initial plan is to drill a tiny hole at each end of the crack to hopefully stop the spread and then fill the entire area/crack with a plastic weld epoxy. I have serious doubts that it will be a long term fix but may get me by until I get a new tank (apparently at least a couple weeks). Another concern is the epoxy I have found will not work on polyethylene or polypropylene plastic and I don't don't know what the tank is made of (another unanswered question for TM).