I would recommend welding ABS, PP or PE thermoplastic tanks rather than using a resin like epoxy. Thermal hardening resins and thermoplastic normally does not go together. There are some "fix kits" out there and I believe they are some kind of acrylics, how long those repair hold up I don't know. I have never seen any of these products sold or used used in any industrial application so I personally don't trust them. This is a job you only want to do once.

Drop the tank, clean it inside and out, drill each end end of the crack with a 3mm drill bit, use a Dremel or likewise to cut clean the cracks 2/3 the depth of the material. Go over the area ever so lightly with a propane torch and/or clean it with acetone to prepare the area, then "weld" using a soldering iron an add material as needed from a plastic welding rod. I bought a "kit" with different types of 3mm plastic welding rods on eBay for a few dollars. I did this fixing quite a few cracks in the grey water tank for #477. It worked really well, and I have tested the integrity by using a hammer beating the crap out the welds because having leaks of any color when everything is reinstalled is not an option.

Picture here  http://www.fmcowners.com/mbbs22/photos/show-album.asp?albumid=89&photoid=4645

Stay cool!

Kjetil #477