The super RV’s in $1M and up class have huge holding tanks. Often there is only one tank with a 150+ gallon capacity. These tanks can hold a lot of effluent for a long period of time and can get really bad. Usually the odor will not permeate the coach because they use toilet systems that seal the lines between the tank and the toilet preventing odor even when being flushed. Left untreated the odor will eventually permeate the plastic tank and be an issue. In our experience these super-size tanks do not drain well. They retain a small amount in the bottom that cannot be pumped out similar to a typical boat tank. The solution here is simple to keep the tank sludge and odor free. Two ounces of TankTechsRx in a clean empty tank will get it started four ounces if it is really dirty. Reduce that to one ounce after each dump and everything will be fine. Failure to do this is equivalent to driving a million-dollar outhouse.