Steve, after posting I came across an image of a Sealand floor flange seal (that I've attached). I'm guessing that's the thing with the tabs. It seems that my old toilet also had a wax ring that did double duty. The wax and other uh, unmentionable material obscured the inside of the pipe and I couldn't tell what was going on. I'm not familiar with RV toilet installations and didn't know if those tabs were part of some sort of rubber collar that extended down the inside of the sewer pipe.
I haven't received the ordered toilet, etc. yet but, wanted to see what I was up against. So, I pulled the toilet, took a pic and put it back. I'm assuming that I should remove both wax and rubber seals and just go with the one that hopefully comes with the toilet?