some rv's have the toilet on the shallow end of the tank. so that the solids build up and plug the toilet drain.. the way to resolve it, is to open the valve and use something to break up the mound and get it to spread into the tank… (i used the metal awning reach rod… which is easy to clean afterwards, i store it next to the toilet)..
you will then need to use lots more water when flushing, until the black water tank gets about 1/3 to 1/2 full.. then cut back on the amount of water use… then when flushing the tank next time. leave some water in the tank. as a primer and it will help rinse the shallow end when traveling…
my next rv, i will make sure the toilet is on the drain side of the rv….