I repaired our grey tank, which is exactly the same as the black tank. They are both ABS, and ours cracked in the molded v-groove on the bottom. I ordered Plasti-Mend directly from the company, and it worked great. I then added angle iron supports across the frame, in addition to the two braces already in place, but did nothing to support the tanks. I then added spacer blocks of varying thicknesses to the tanks which are supported by the braces. I have heard some just add a sheet of plywood under the entire tank. My post with photos is on page 6 at this link:
http://www.montanaowners.com/forums/...ms=tank,repair