i found out that some tank encounters are scripted. :-/ i was playing the final level of blood harvest with a script which removes special infected & bosses, so there would be only normal zombies. i wanted some night of the living dead feeling
everything was fine but the rescue team never arrived. after almost 20 minutes i deactivated the cheats and finally a tank was spawned. we killed it, another one was spawned, we kiled that one too and THEN the rescue team arrived. so it seems that there are always 2 tanks and if they can't get spawned then you will fight endless waves of infected.
should be same for other levels. i thing in the final level of no mercy there are also always 2 tanks before the helicopter arrives.