"To return to my explanation, the black tank IS effectively a closed system up until the point the liquid has risen to the top of the vent pipe."
As long as a tank is vented, it is not a closed system. If at some point the vent is blocked, the tank then becomes a closed system. That action causes pressure to build in all directions as a stronger force introduces volume (water) into that space. Just like a hydraulic ram.
I agree in that with what we know, the tank failed before any fluids reached the roof. However, that is insignificant. The tank failed by the waters weight exerting static pressure downward. Not by an excessive hydraulic pressure exerting pressure in all directions within the tank.
BTW, Happy Thanksgiving.
2016 3160, Legacy, Sailuns, Splendide 2100 xc vented, 1 1/2" axle lift blocks, 2014 Ram 3500 SRW SWB 4X4 6.7 Aisin Mega Cab, EBC slotted disks and brakes, Titan fuel tank.