With regards to black tanks, our 28a fills the black tank frequently since it's tied to the bathroom sink and we do a lot of hand washing, teeth brushing, etc. We do regularly fill the grey tank and have a "blue boy" in case we need it but it's never been an issue for weekend camping. If I was boondocking for a week or more then the capacities are woefully small for both tanks.
As for the 19g model, why wouldn't anyone just plumb in separate waste valves for each tank? Seems like this would be an easy way to solve some issues.
