After spending 2 weeks at Quartzsite dry camping and observing how long term dry campers on Bureau of Land Management land survive, I can tell you that you need to look into what are called "blue boys". The people that live on BLM land for 6 months dry camping have all purchased the blue boys which are a blue plastic tank on 2 or 4 wheels. Either type have a handle to pull in some way. Some pull them by hand, others pull with their car, others pull with a 4-wheeler, but you drain the holding tank into the blue boy and then take the blue boy to a dumping station (nearby) and dump the blue boy and leave the coach sit. Now, it all looks like a pain in the butt to me, but that is the way that they do it for long term dry camping. Others arrange for a "honey wagon" to visit their coach and drain their tanks every X number of days or weeks. Either works for those that do it.
Tanks in an Entegra (and I think everything else) are stacked. My 100 gallon fresh tank is on the bottom, then the gray and black are stacked side by side on top of the fresh tank. All are in a compact compartment. On my 15, the black is I think 50 gallons (50/50), but on the 16s it says the gray is ~60 and the black ~40. As other have said, use of the half bath toilet is very water efficient.... the back bath mascerator not nearly so much.
Gary and Dee, Zowie and Bowie (traveling cat sibs)
2019 Cornerstone 45B, X15-605hp, Imperial, Spartan K3,
2013 Honda CR-V toad, Demco Excali-Bar II,
Demco Baseplate, Demco Toad Light system, 73 de W5FI