Yep, if you just want to put an ammocrate inside a tent, it's a matter of putting the tent down and putting the ammocrate on top of it, then previewing to see if it's where you want it. You'll probably need to move it around a little to get it just right, but that's all there is to it.
Now, to put guys in watchtowers or put radios on top of tables, you'd need to use the setpos command.