Wow Phil that's a scary recommendation. if he pokes upwards with a plumbers snake I can't imagine the mess of the effluent coming down in a full tank. I think if it is the valve, remove the undercovering to get to the valve and see if there's some way to get it slid open.
If the valve is open but there is a bolckage and if I was going to use a snake I would buy an extra down spout sewer fitting and knock out the plug where there is a hose bib (look at this example):
http://www.pplmotorhomes.com/cat/898/250/898442.jpg
Note the hose bib. Then I would connect a hose to the fitting and hold the hose up higher than the black tank by a few feet, then use the snake all the way down the hose and up the drain hose to the black tank (good luck getting it finding a way to get it there). Then if you unplug the blockage and the effluent comes rushing down and you hold the hose up high hopefully the down rush will go into the sewer drain and not all the way up the hose. That's why the hose has to be up in the air as high as you can hold it (you will probably need a helper).
Otherwise it's Rid-X, or some other ways to dissolve the clog or a call to a professional plumber if that still doesn't work.