Crawl under the trailer and find the big 3" or 4" drain pipe coming out of a holding tank. Follow that pipe to the end of the pipe. Where the pipe ends there should be a handle connected to a valve. Also near the end of the pipe there should be a smaller - maybe 2" pipe - that joins the bigger black water pipe. My setup - and my previous 2000 Sprinter 5er setup, had the black and grey water pipes terminating at a dual valve setup - a big valve for the black water and a smaller valve for the grey water, both feeding into one connection for the sewer hose. Drain the black water first, then close the valve and drain the grey water. That will wash the nasty black water out of the sewer hose. Then disconnect and rinse the sewer hose with fresh water before you store the sewer hose.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.