Thanks everyone - I went back out and did a bit more investigation. The smell really seems to be coming right from the toilet itself. The slide/seal it working and holds water and I looked all around and couldn't find anything out of the ordinary. It's been about 6 weeks since I parked it after the last use. It's possible one of my kids "used" it and won't admit it. Even so, I've had stinky black tanks before and isn't like this.
I'm heading out tomorrow and will be at a full-hook site by evening. I'll dump, flush and clean everything the best I can and re-evaluate. I'll either be posting an update from the campground or the nearest Holiday Inn because there is no way the DW will sleep in the thing the way it smelled tonight :)