[folivier]

If there is room in the gray plumbing to add a fitting either a y or tee so you can divert the water then you can use one of these to go into the tank. I used a 1" one to add a water fill to my tank. Easy to install using standard pvc pipe. They have different sizes available:
https://www.ebay.com/itm/White-UNISEAL-F...0010.m2109

https://www.ebay.com/itm/White-UNISEAL-F...0010.m2109

[Guy]

    Here is what I assume was the Newell soloution to having only 1 tank for grey and black water. Note when the 2nd valve (grey water) is closed, the water fills up the pipe and enters the tank .

[folivier]

Looking at your pictures if you reverse the 2 dump valves position, IOW open the closed one and close the open one, the gray water should go into your single tank. Then open the dump valve that is closed when you want to dump. That is basically how my '93 and '98 was set up.
Aren't you glad you're not up in the Yukon trying to figure this out?

[Glenn and Laureen]

This is my second attempt today to post a picture but am not having success. Will try again later tonight . I do appreciate the responses and understand the logic of opening the vavle to the waste tank and letting the shower water flow into that tank. Seems like an inelegant solution to me but I value functionality over elegance at this point.  Will not make further comments until I can post a photo.

[encantotom]

that does not look like original plumbing to me. i bet it has been redone for some reason.

should be no reason you cant do it to make it work the way you want.

tom

[Glenn and Laureen]

Photo of my plumbing is attached. Black pipe in center comes from shower/kitchen sink. Black pipe in upper right comes from bathroom sink. Pipe from toilet is not visible in photo but is behind the pipe from bathroom sink. My two paddle valves are visible. Short black pipe going to the left off the center pipe is sealed and goes nowhere.

[Guy]

Well there you go ! They did use a cookie cutter approach for the basics back in the day. That pvc stack is a clone to the one in our coach, so fo the most part, you do have factory plumbing . As to the thought of trying to drain sink and shower water into the black tank by reversing the two dump valve positions, I don't think we should try to fill a black tank from the bottom. Being old school, this black tank has a toilet sitting on top of it, so we know what is in the bottom of that black tank.

Your descriptions of pipe functions make me think you have a streetside kitchen? Ours is drivers side, so that is a plumbing difference. That blanked off left pipe was for a drivers side kitchen sink .  I would think that the center pipe was meant to drain in  the top of the tank, but the left pipe could be redirected to do do.  With the 4 gallon per minute water pumps, Newell did not see a need to vent the drain stack . The waste tank is vented by what we would consider to be an "oops" overflow pipe coming from the top of the tank and out thru the  bottom of the water bay
Now we just have to figure a way to terminate that left pipe into the top of the tank.  I want to believe that you could cut out a hole in the top of the tank and insert a male pvc adapter since it is gravity application. 
If this system has been hacked by a prior owner, there might be a blanked off hole on the top of the waste tank?

Now, isn't this easier than electrical ?

[Richard]

Ok, the purpose of the valving setup is to allow one to dump the grey water on the ground and keep the entire waste tank for black water IF you were in a situation that warranted that. Otherwise keep the valve closest to the tank open, and the discharge valve closed and you will be fine.

As far as putting in the grey water at the bottom of the tank, all I can say is that in the 12 years of Newell use, it has not once caused a problem.

And I haven’t inspected the roof of this coach, but I would be surprised it there weren’t a vent pipe on the roof from the waste tank.

[Glenn and Laureen]

Thanks everyone. It's humbling to learn Plumbing 101, Electronics 101 and Air Systems 101 on the fly in front of folks who know what they are doing. I appreciate everyone's comments. The good news is that I AM learning!

Guy, our kitchen and shower are curbside, toilet is driver's side.

[Glenn and Laureen]

To follow up on this series of posts, I thought about what everyone said about how the plumbing is supposed to work, considered adapting my plumbing to add a discharge from the shower/kitchen line directly into the grey/black tank but decided to just let it work the way it was designed to function. I opened the valve to the grey/black water tank and let the water from the shower flow in. Guess what? It worked. No messy gunk coming up in the shower pan, no unpleasant odors, no bad anything. So, yea physics! Thanks to everyone for your advice.