Water running uphill..?

[BradKW]

Well you could pump it, but I always try to avoid that. Just one more mechanical system to break. One thing that may help is you can buy a compact trap with a drain hose attached  at an RV supply. They are usually listed as a pop up sink drain and screw directly onto the sink strainer so you eliminate the height of the tailpiece. If you go to my cargo trailer build I think there's a picture of mine in there someplace. Other than that, I'd relocate things until it works. No sense being g so tied to a layout that it causes the basic mechanics to not work. I'm always a fan of form following function.

Ok, looks like some redesign is in order. Couple other questions:

I got a couple of these [img=200x200]https://images-na.ssl-images-amazon.com/images/I/31llU2X05sL.jpg[/img] "waterless" drains by Hepvo, any opinion on them? 
I figure that for the shower it's necessary because there's no room for a Ptrap, but under sink there's room for a convention P-trap if it's better?

Venting: I figured I'd use a interior vent for sink [img=100x100]https://images-na.ssl-images-amazon.com/images/I/61U2WVopcrL._SL1500_.jpg[/img], but how do I vent a shower drain? Also, what are my options for venting the tanks...does each line need to go up through the roof? Could freshwater tank vent just be a open and hidden in an interior cabinet? And what is minimum size for vent pipe diameter? 

Thanks!

 

[AngryVanMan]

The waterless Hepvo traps function as air admittance valves when the drains using them are not covered, which is basically the same thing those Oatey Sure-vents do. As far as the Hepvo valve/trap quality, I've never used one personally, but I planned on using them in my build fwiw.

As to your uphill pipe, depending on how high bottom of the sink is relative to the high point in the pipe on the other end, this may be of little practical concern, as the distance between the bottom of the sink and the top of the uphill side of the pipe may generate enough head pressure to make the uphill section function as a slow p-trap of sorts. The length of 6 ft. uphill is a little concerning because that introduces a lot of friction which will make the drain flow slowly, which increases the chance of obstructions forming in the pipe over time regardless.

The solution is fairly obvious. You must install a F-series Superduty 4x4 axle under the front end with associated conversion hardware, transaxle, etc. This will fix the rake in the suspension and is clearly 100% necessary in this case.

 

[masterplumber]

I've never used the Hepvo drains, so can't speak to their reliability. I'd avoid the one in the center though. Any corrugated flexible drain collects crud in the corrugations, which then becomes moldy and can also cause clogs. I use the Oatey style vents all the time and they work well, they are generally 2'' with an adaptor to 1 1/2'', but you can order 3'' and larger also. 1 1/2'' is probably the best for your tanks. You could get away with less as you don't have much flow volume, but 1 1/2'' is a standard and available size. Stay away from the little black "sure vent" or "pro vent" as they are prone to failure. That is why they are only approved by HUD for mobile homes and not approved in the building code. For a mobile dwelling I wouldn't have an issue with only venting inside, and not running a vent through the roof. You are not tied to the sewer and empty the tanks often enough that even if an air admittance valve failed there is not much worry about toxic fumes or smells. That of course is for gray water - if you plan a black water tank I'd definitely vent it to the outside. My tanks are actually portable Reliance containers, so I just leave the little vent on the container open and have never had an issue. It's only like a 1/4'' opening. As for having a pipe running uphill and acting a a "long" trap, in theory it can work. But in reality it will be a place that can collect particles and eventually clog. And unlike a trap which can be taken apart and cleaned - it would be a bear to clean out once the build is done. And even "clean" water has some particles in it that will settle out in a slow stream - if you don't believe me set a glass of tap water on the counter for a couple days and then look in the bottom of the glass.

I hope that all helps, mainly when designing, try and remember what you're trying to do with each step in the simplest terms (drain water, allow drainage by venting, etc...) and choice options based upon simplicity and availability.