I know this may sound simple but I had the rod to the slide valve come unscrewed from our black tank handle once. I thought it was broken but just had to thread it back on and bingo it worked. Hope its something that easy for you.

Lindy has a good point.  The last time I drained our black tank, I was inside while the tank was draining.  Before I went back out, I looked at the lights on the gauge and they showed 2/3 full.  I went out and insured that nothing was draining through the short clear plastic sewer adapter and then turned on the water to use the black tank flush sprayer.  After flushing and finishing my "procedure" for flush and drain, I checked the gauges again.  This time, there was only one light lit.  The flushing likely cleaned off the sensors to where they were more accurate.

I don't normally check the gauges, but at least this time it taught me a little bit.

Terry

another way is to purchase a cheap closet snake....and unbend the 90 degree elbow to enough so it will pass through from inside..Note:.(this will only work with direct drop to tank systems)
lower the snake into the tank and extend it into the drain line and crank the clog out

you will have to go out and look at the tank to orient yourself with the drain line first

Mmmm.... just thinking what folks will think when I say what I'm thankful for on thursday! An unclogged toliet! :)

Ive heard that calgon works. In fact I bought some but haven't used it yet. Excuse me, I'll be right back! :)

All is good! we moved nearer to an rv service center and the move, moved things along!!!!! No more clog. Lesson learnt! More water, less paper, and more chemicals. So thankful!!!