The tame tank would randomly appear in lieu of a regular tank. This tank is not aggro to the survivors unless they shoot him first.

He does still charge toward the survivors, but only because he's so pleased to see them. This unfortunately is indistinguishable from regular aggro tank behavior, making it hard for survivors to know.

If he manages to get close to the survivors without being aggro'd then he'll just stand there chilling with his new friends, roaring in happiness.

In cases where two tanks spawn and one is tame, tame tank will attack aggro tank to protect the survivors. Insert confused aggro tank noises here.

If the survivors manage not to aggro the tame-tank and the escape vehicle arrives, they take the tank with them and name him Keith.

Thank you for coming to my ted talk.