I vote for possibly not using enough water. We also use the trash can for the "gently used" paper. Less paper, MORE water!
The use of Calgon water softener (or like products) has been useful for a lot of us. Put in a bit with 3 to 5 gallons of water in the tank before travel. Sloshes around, cleans tank. If you have a lot of sludge built up, you might need professional help. Good luck, let us know your fix.