When boodocking as Jefe has said, sometimes it is a trade off when dealing with liquids. We find that our 44 gal. fresh is the limiting factor in the time spent away from services if you camp in a dry area. We usually carry fresh drinking water with us in the crew but as we are active every day and have short showers,you will eventually run out of fresh. If you camp in wet areas where there are lots of creeks/lakes, then this is less of an issue if you refill your fresh tanks manually or you sometimes take baths in the lakes or waterways which is what we do if camping in our neck of the woods.(British Columbia).
Grey water (18Gal)unless forbidden by law,can be dispersed in remote areas to the trees/vegetation/cactus who usually have their tongues out and appreciate a little drink.
Black water(cassette)can be emptied at any pit or regular toilette or dump station. If you are care full and I don,t want to go into too much detail, it takes a fair while to fill up the 6 gal portable tank. I was a little concerned at first about this option(cassette) for boondocking but have found that having to find a dump station to dump your black water tank when out in the boonies for awhile is not that easy.
Having limited capacities except maybe for fresh allows you to conserve and ultimately lets you stay out longer in the remote places that you appreciate.
Best of luck in your TC search.
Dave