I have a keystone Laredo 29rk my wife does about 3 load of dishes and the tank is almost full is it smaller then the gray tank for the shower and sink up stairs
Use paper, and use less water when washing dishes..don't let the water run continuously when rinsing. If you aren't on full hook-ups you can't wash dishes like you do at home.
If necessary... I also use a RubberMaid dish pan that fits in the sink.....then carry this to the toilet and dump it in the black tank...Lots of soapy water is good for the black tank and saves room in the galley tank.