The black tank in our '72 Argosy 20 was in such bad shape, we eliminated it completely and installed (?) a Thetford Porta-Potti instead. Since it's just Liz and I, it was a pretty good option. We don't boondock much, so if it's a #2 trip, we tend to use the facilities at the campground for "the heavy stuff".
Some people are like Slinkies. They're really good for nothing, but they still bring a smile to your face when you push them down a flight of stairs.
TAC - NJ-007