This is the vent:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
It works really well. It's important to have a good sized vent so that when you pull the handle to dump, air replaces the volume that is evacuating the tank at a rapid rate and no vacuum occurs inside the tank. That gives you a nice clean "Dump". Another trick is to have someone hold down the flush pedal on the toilet while you open the gate valve outside.