What is the capacity?
How many people in the coach?
How do you know its filling up? Are you basing that on the panel lights/gauge or do you see waste when you look down through the toilet?
If going by the panel lights/gauge they are famous for being way off.
If it is truly filling in a day that means you may have a mound of solid waste in the tank from the previous owner.
Some owners will leave the black vale open, allowing liquid waste to drain and solid waste to be left behind to solidify or they might drain too soon and cause the same issue. Black tank should be drained somewhere between 2/3 to 3/4 full so there is plenty of liquid to move the solids out.
If there is a mound of solid waste you you will need to break it up and flow it out. Try filling the tank through the toilet with several gallons of hot water to break up the mound. Let the hot water soak for 30 minutes or so, repeat.
Follow this link for more reading on black tanks https://www.google.com/#q=black+tank...com&gws_rd=ssl
US Navy Vet, Liberty Tree Member of Oath Keepers, NRA & VFW Life Member, Alaska EMT.
2009 Safari Cheetah 40 SKQ
2009 Winnebago Chalet 231CR