My valves, from front to back, are black 1, gray 1, black 2, gray 2. Maybe the reason HL doesn't label them at the factory is they can't keep them straight!
We typically drain the tanks every time we leave a park. We haven't gotten into boondocking yet, but will likely find a dump station after every trip, so nothing would ever sit in there for very long. We use laundry detergent and Calgon, and have never had a problem with smell. If you use chemicals, I believe you should put them in before you start using it.
I'm seriously considering that the rear lav only be used for #1 from the kids when we are only out for a day or two. That would make dumping a lot easier, because I wouldn't have to fill that rear tank to dump it.