On my '77 Safari, removing the bumper and trunk give me good visual access to the tank drain valves.
Remove the bumper - 4 bolts,
Bend the belly pan down
Pull the Plastic Trunk out - there's a drain hose you have to pull out with it.
This will give you visual access to the tanks and valves, if you need to do repairs you will have to pull the rear belly pan. Sorry I don't have a picture.
1977 Safari Land Yacht
2005 Toyota Tundra SR5
2010 Ford Flex Ecoboost