Well, I am still not sure what type tank we are talking about!
This is a long thread and we still don't know the model number or type of vehicle.
The discussion started out fairly generic, but now we are getting pretty specific (as well as MODEL specific).
There is another thread here somewhere that goes into the flushing process in far more detail than we have attempted here.
If the tank will not DRAIN completely; there is more than a poo problem. If you look at replacement tanks they ALL have sloped bottoms to the low point drain. Either the camper is not level or something is BLOCKING the drain valve preventing it from emptying (or opening) completely.
Could someone have dropped a small towel or pad into the bowl?
This is a MUCH different issue than the tank INDICATING it is "not empty". Blueeyes stated she sees WATER in the tank after she dumps empty. This seems to indicate her hopper is straight plumbed into the tank.
FYI - My down pipe from the hopper has 2 45 bends in it and I can't see inside the tank from the hopper valve.
If it is an indicator problem I will post the troubleshooting links for that system. However, since it "smells" I tend to think this is a blockage issue.
Fishing around with a coat hanger made into a "hook" might be in order.