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?

Your kitchen will go in as 2" with its own vent out the roof. Make your shower 2" aqnd the lavatories at 1 1/2".
Your best bet would be to strip off the undercoat to see how the drainage's configured. For a single bathroom vent here's how it will rough in. Most bathroom groups are roughed in like this.
Toilet connects to sewer main. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof.. The toilet wet vents through the lavatory vent and the shower connects to the lavatory drain and is wet vented by it. The vent off the lavatory may be run out the roof to service the entire group. Good luck, Tom

Thom
Can I run a 3" main drain from on one side of my trailer and install a 3" to 1 1/2 " tee to vent up and out the existing roof vent hole. The 3" pipe will contine back were it will drain to the septic. The shower drain will tie into the 3" main then the toilet will tie in 48" away. The vent will be after these two tie ins. The kit sink will be on the opposite wall and will drain down twards the middle of the trailer and turn toward the main 3". The bath sink will come from the back and meet the kit drain in the middle of the trailer, tee off and connect to the main 3" drain in between the shower and the toilet. This will give me one vent so that I would not have to cut another hole in the top of the trailer. The vent would be at the end of the run as the waste water drains to the septic. Just not sure if this will work, I have concerns of gas smell backing up into the shower and sinks. I do plan on installing traps at the shower and both sinks. All this plumbing needs to be installed under the floorand that is another challenge. What about some type of flabber valve at the end of my 3" drain that would only allow flow towards the septic then close when now flow... any such monster... any suggegestion to help out a confused want to be plumber.

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I need more clarifacation

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Ask away! What do you need clarified? Are you stripping all the plumbing out and doing it over or are you going to use the under floor plumbing? Cheers, Tom