Thanks for the tip ticki2, I'll keep a look out. Yesterday, after seeing your post, I was back over to the store to return some parts and get some new ones. I looked then for the device you described, or something like it. Also checked at the RV parts store on the way home and they no longer carry it, but can get it. Specifically I wasn't looking for something like the anti-siphon to customize, but rather a tailpiece with the basin nut as part of it. Will keep looking, as I think something like that would be a great replacement for what I have now.
Regarding your question, the stopper is neither screw in or push in. Maybe pressed, but I didn't want to risk damaging the o-ring trying to determine for sure. There is a concave groove all around the top of the drain piece. The stopper is a convex shaped disc that fits into the groove. The o-ring goes around the convex piece. So the stopper can twist and move about in any direction except out. Not sure how it was put in there.
Here is a close-up.
I tried your other suggestions and for the most part things worked very well. The one aspect that didn't was the Fernco Ell onto the tank stub out. The stub out is a great idea, as it makes it much easier to repair leaks without manipulating the tank, but the swing of the Ell came out too far and the door wouldn't fit back on.
The next thing I tried was a stub out including elbow, using a reverse installed street elbow, it spun in without disturbing the tank. I had to remove the door catch and re-install it, but otherwise, a slick job. This photo shows the end result, a much better final design than anything else I've tried.
I'm now going to move the discussion over to "Waste Water" to finish up this subject and today's post.
