there are lots of places to dump "blackwater waste" but not letting it build up is an important habit. commercial campgrounds may let you dump for a small dump fee, but it can be more efficient to book a day in to clean up purge and tidy things. There is a technique called "cat holeing" find an out of the way place deposit the dumpling in a shallow hole 6-8 inches from the surface, include the topsoil and organic debris, and soil microbes decompose it quickly. Do this at least 200 feet from water for sanitary reasons and all is good for practical sense. There are ordinances of aesthetics regarding dumping, i very often include a few apple seeds in the hole, in the high probability there will be a well established tree for all to partake in, human and beast alike.