Black tank repair?

I am going to give you the CORRECT answer you will not like. Based on 38 years and still working on Allegro motorhomes and have replaced at least 100 of the old 1976 thru 1990 underbelly Allegro tanks. THERE IS NO REPAIR YOU CAN MAKE ON THESE OLD ABS TANKS THAT WILL HOLD. THE ABS IS TOO WEAK AND FLEXES TOO MUCH TO ALLOW ANY TYPE GLUE/WELD/PATCH TO STICK. EVEN IF THE TANK WAS ONLY 1 YEAR OLD. YOU have the time and luxury of attempting to do the impossible. But, you are slowly learning. It takes a professional less than 4 hours to replace either tank on the old Allegro's. Have you called Tiffin to see if they still have your model tank? Doug