You seem to be mixing up gray and black water disposal.
There is a way to handle this if you are not near a dump. Use less water. When I boondock in dry areas, I get by with 3 gallons of water a day. About half of that is for drinking and cooking. The rest goes to a shower. When my wife is also traveling we are still barely twice that amount. So maybe 5 gallons a day goes into gray water. I don't have a black water tank. I have a cassette toilet. It needs to be emptied every few days but I can use a dump, outhouse or even a regular toilet.
People who need a lot of water also need to have hookups and frequent access to water and a dump.