3650BH - Black Tank issues

[bucketman69]

New Gateway 2015 3650BH - went to back flush the main black tank first time filled 3/4 full, went to drain and heard two loud snaps. Turned out the PVC pipe going into the tank snapped and the 1 1/4 PVC pipe coming out to the main wast gate snapped and the entire tank sank through the bottom.

 

[danemayer]

Am I understanding you correctly - you were using the black tank flush and then opened the gate valve to dump the tank?

Is it possible the tank was closer to full than you thought and perhaps was over-expanded by the water from the black tank flush connection?