How long a tank will last will depend partly on it's shape/configuration. Some tanks are quite shallow and "solid" waste doesn't spread sideways like water does. Our blank tank is shallow and the most we can get out of it for 2 people is about a week. Had a previous TT with smaller tank that would last longer due to it's shape.
Don't skimp on water to make the tank last longer because it won't. Add a few gallons before using it and don't skimp each time you flush or you can get the infamous "pyramid of poo" in just a few days.
