My 77 Safari has 2 ways to drain the fresh water tank.
Inside the cabinet under the bathroom sink are valves that drain outside underneath the rear of the trailer, they are part of the freshwater system so the pump runs until the tank is empty.
The other method is to crawl under the trailer and remove the access panel in the tank cover, there's a valve inside that will let the tank drain by gravity.
1977 Safari Land Yacht
2005 Toyota Tundra SR5
2010 Ford Flex Ecoboost