If multiple units have Tank does the attacker decide the order of damage?

Yes — the attacker chooses which Tank takes damage first.

When multiple units with Tank are on the same side in combat, they share the same damage-assignment priority, so the assigning player (the attacker, who assigns first) may pick any order among them. Units without Tank can't be assigned damage until every Tank has lethal damage assigned (Rules 815.1.c.2, 460.2.c.6).