E.C.M.,
here is some info. At least we can narrow things down as to what doesn't work .... may as well skip those steps .
From what i have seen in the past, people have ruined the tank and/or fitting by using improper cleaners to prep and sand the working area .
Most of this stuff sets up fast and once its done, it might be DONE.
It might be best to take the advice of one of our members 'Golden HVAC' (see the link below) to call one of the plastic MFG.'s and be guided to what they tell you .
If you do a search on 'waterproof pvc glues' you can read up the mistakes that can be made.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/25104762.cfm