Full black water tank

In theory, there should be no fear of "collapsing" the black tank by sucking the waste out, either through the toilet or through the dump valve; the roof vent should prevent that from happening. Perhaps the suction is so strong that it overpowers the ability of the roof vent?

I've had my black tank pumped (hooked up to my dump valve) a couple of times at Glamis; no issues whatsoever. That's all they (the honey wagons) do there (empty RV tanks) so perhaps they are well aware and use a reduced suction?