The black tank on our 61 tradewind is about 10 gal. based on pouring jugs in it till full. We have a grid of pipe under the trailer which we plumbed the gray into which is also about 10 gallons.
We can stretch the black tank up to 6 days with care.
Hardly seems worth going the composting toilet route for a gallon per day per person or less.
We will use a 10 gallon blue tank to transport "waste" when a dump station is available. A 5 gallon works well at 2 day intervals when dumped in a pit toilet.
The small tanks can be lifted and carried on the tail gate. The big tanks have to be towed to a dump station. Not always practical where we camp.
The above the floor black tank will not work well for gray water, of course if you leave the plug out of the bathtub drain that will fill before the tank increasing your capacity