As far as triggers are concerned, the only units inside them are living ones.
Even if they're set to:
Anybody PresentSo if you place a tent inside a trigger, the trigger won't see it. Similarily, a trigger cannot see map entities (trees, buildings, bushes etc.)
Short of making a looping script that goes through every object on the island (all half a million of them
), I don't see how you can get a list without entering all the object ID's manually into an array... sorry
Maybe there is some smart cookie out there who has discovered a cunning way of solving this one... but I'm afraid it's beyond me.