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Bought a 1977 hi lo to restore. Shouldn't say restore more like make it how I want it. Its not great. Plan is to make it great.

Started tearing it apart

looks like a cool project. Lots of people on here have done the same. Good luck and welcome.

Rick

Pulled it out of a field. Owner said he loaned it out a few years ago and just got it back. I had to crawl thru the trailer to look inside Nothing was working. Its not been taken care of for a few years. New solenoid, wires, battery and a new switch. It now goes up. Its got new cables and adjustment is good.So far roof leaks and its rusted out over a window.Bathroom looked worse then any bar I've been in. My wife said NO loudly so its gone The Rest we will see. Got an owners manual off this site its been very helpful.

MrEd, If you haven't already, check out the videos (37 of them!) on youtube. "Hi Lo operations manual", something like that. Tells you the basis operation of each system. Might be "Hi Lo owners manual".
Tree

Has anybody gotten rid of the vent pipes going thru the roof. Thinking of using these instead. PVC Air Admittance Valves

Yes the vent pipe can be deleted.
A negative pressure vent must be installed at the top of the vent stack in the cabinet.
ALSO NEEDED....A second vent line from the top of stack vented out the bottom of the trlr. The second vent is because the drainage system is closed and produces its own gases and pressure. I used a flexible plastic tubing and ran it next to the main stack and out through a small hole in bottom of trlr. I just drilled a small hole and siliconed it in place, and at exit hole also.
I am/was a plumber and it works great.

Thanks I thought I could. How big a tube I'm thinking small. Can't stand those 2 pipes sticking up thru the roof. Sounds like they should of hired a plumber in the design team.

I eliminated our vent too with a negative pressure vent under the counter. Works great, and 1 less hole in the roof to worry about (I totally rebuilt our roof). I eliminated the toilet entirely. So no need to vent it haha.

From memory i used a 2" neg pressure valve for the top of the vent stack. Use the size needed for your needs, 1 1/2" will work just as well. Used a 3/8" plastic tube for 2nd vent.

"Sounds like they should of hired a plumber in the design team."
The way we are re-configuring the venting system would not be allowed in the manufacture process. They have restrictive codes that must be met.
We just know how to fix things to work correctly.

NMALOZZI.
Your negative vent needs to be raised to the highest possible point under the COUNTER. You are below the water level of the sink.
With drain use, it will send water level into the valve and cause failure.
Move valve to side of sink to achieve.

Going to vent both under dressing cabinet (its the tallest)Pressure Vent will be above both sinks bath and kitchen. Toilet vent is going there also.

You only need one neg valve for entire trlr.
That is the best place.

Thanks renoites

I think the angle of the photo is making it look odd. I assure you it is as high as it can go. The neg vent has less than a quarter inch between it and the counter (I needed that quarter inch to be able to get everything in place). We've used it multiple times now with no issues.

NMALOZZI
The pic shows neg valve under sink not counter.
It will work, until it gets junked up. I do realize you have deleted other drains and this is not a big issue.
Just thought I would point out correct install for others.