Both water tanks were bought from Dura weld. Now about vent fitting hole... First, I was surprised, Ronco made it on the side of the tank, but during installation I quickly realize that it was done with purpose. After you install barbed fitting for waste pump out, look through vent hole to inside of the tank to see if your pick up tube still located vertical, because while installing fitting it can be turned to one side or another and then you won't be able to pump your tank empty. Pay great attention to that, or you'll be carrying some waste all the time and will be surprised why tank is full so fast. Also, check if your vent hose not clogged before connecting to the tank-very important.
As I said before, I bought Shields Poly-x hose for my waste tank plumbing and I'm really happy I did. The hose is really flexible and easy to put on fittings. Most expensive hose, but really the best.!