exactly that!
in your briefing where you made the objectives, they should be numbered. so have a trigger that activates on the condition that the tanks are destroyed:
(getdammage tank1 >=1) and (getdammage tank2 >=1).... etc
then put this in the activation field of the trigger
"1" objstatus "done"
if blowing up the tanks was the first objective in the briefing of course