Your problem can be hard to clear up, but let me give you some suggestions. Pour a large dose of the chemical used in Black Water Tanks to help break things down, and then pour Ice Cubes in the tank. Once done, drive the camper for 10-15 miles and let the ice shake around and hopefully break up more of the poop. Then let the ice melt and drain the tank.
You may have to do this several times.
The only other way to take care of this kind of problem is to hire one of the professional tank cleaners that you can often find serving local campgrounds. They use high-pressure steam and essentially break up the Poop and then clean out the loose debris.
On another note, always make sure you are using toilet paper that is designed for septic tanks and camper tanks because it breaks up quickly when wet.