How do you know the tank is 3/4 full?  If you are depending on the idiot lights on the panel . . . don't as they are notoriously inaccurate.  When my black tank is empty it sometimes reads 1/3 full and at other times 2/3 full - not dependable. 

If you can look down the toilet and see liquid than chances are pretty good that you have a plug at the tank exit - open the drain and insert a stout stick or similar to break the plug loose.  If you cannot see anything down the toilet, for peace of mind, open your drain and then take a garden hose and insert it inside the toilet past the lid cover, turn water on and flush tank out that way.  If you are confident that the black tank flush works use that but make sure your drain is OPEN while doing this procedure.

Listen to Jato advice.

One thing every camper owner should consider owning and having with them at all times is what is called a "sewer or toilet wand". This is simply a 4 foot section of 1/2 PVC that you connect to an on/off water valve that you connect to a water spigot hose. I carry mine inside my extra sewer pipe carrier inside the pipe. If you run into the problem of a clogged black water drain you can open the toilet drain flap and insert it into the tank and blast the clog free. Plus its great for flushing out the tank after a long trip or the end of the season winterizing clean up. I've never had to use it to blast a clogged up drain free but I did let my buddy borrow it for the same problem in his motor home rv.

Hey Gang,  Why can't Sharon68 use the Reverse Flush Valve?  You can probably barrow one from another campsite, or purchase one at the local camper equipment store.  Don't you think it would sort of juice up the stuff from the bottom up?