Well, if it's PVC, then the choice is easy. It probably wouldn't hurt to give it a try. If it sticks, you found a solution. But my guess is that it is not PVC; if it were, it wouldn't have come apart. PVC cement is not actually an adhesive -- it's a solvent. It's softens the 2 pieces, and when they "dry", they become one piece. If you glue a PVC pipe into a fitting and try to pull it out after it sets, the pipe will break before it comes out.
If it's not PVC, my guess is it's polyethylene. That is what the rest of the toilet is made of. If that's the case, this is the only thing I've seen that even has a shot at working:
http://www.stealth316.com/2-dp8005.htm
...but 1) it's expensive (like $80 or so for a tube); and 2) it's effectiveness isn't 100%. Even the the Thetford engineer I talked with several months ago said it was "ok".
That seam could be plastic welded together, probably pretty easily. Might be worth a shot to take it to an auto body shop to see if they could simply weld it together. Might cost you $20, since there is no polishing or sanding that would need to be done.
Dave