Dometic toilet blocking in the down tube

Keep in mind an RV toilet is not like your house where there is a gallon of water ready to flush the waste away. It's up to the toilet user to make sure enough water goes into the tank with the solid matter. Holding the valve open as you make your 'deposit' doesn't really add that much water to the mix and puts a solid pile right under the drop tube. The guaranteed way to keep things flowing after toilet use is to move the lever/pedal partway to fill the bowl without opening the flush valve then, once full, move the control fully to open the flush valve dumping the bowlful of water in one big slug. This 'water impact' will break up the solids and disperse them in the tank. Even a second flush like that is a wise idea. Try to get all the users into that routine and you will never have a problem. Of course this doesn't work if there are already 5-6 pre-existing piles blocking the drop tube. In that case, you just have to deal with that and get things moving first.