I have a 1979 Airstream travel trailer that I want to install standard house plumbing, fixtures,drain and venting. The trailer has one hole in the roof that appears to have been a vent hole at one time. My problem is that I do not know how to install and tie in all the drain pipe and how to vent it. I was going to use 3" main drain pipe and use 1 1/2 " abs for the two sinks and one shower. I did find some short 3" air vents that were left in the trailer when I purchased it. They appear to be used near the old existing sink and shower location. All the original plumbing was taken out so I am not sure how that was installed. The trailer will be set in place for a few years while I build my cabin. It will drain directly into my existing septic tank. Can anyone help with the lay out?