As water is taken from the tank to your pole by the pump, if you do not have anywhere that the air can escape in the resulting vacuum thats left in the tank it will suck the sides in of the tank. The faster the water is drained the larger the vacum the greater the suction effect.
If you allow the air to escape from the vacuum thats created then no damage to the tank will occur.
Many overcome the vacuum effect by drilling tiny holes in the tank lid, The only problem with this is if you are fully loaded with water and park on an incline as your first job of the day water will come up through the holes, so the tank lid is no longer 100% watertight.
You can overcome this by drilling a hole thats just big enough to put a small pipe insert through from the inside of the tank lid and attaching a piece of 6mm pipe to the insert and securing it with an O clamp. Then insert a T splitter in the 6mm tubeing to stop any particles from falling into the water.
I will post a pic later today of how I did my sons vent.
Nel.