If it won't drain it is plugged and as rscraig mentioned a back flush adapter is the best way to start. Fill the tank at least half way so it can maybe soften up the blockage. I never leave my black tank dry, EVER. There is always at least 5 gallons of water in there. I don't even drain it dry in the winter. I will drain and rinse thoroughly. Then before putting it away for the winter I add a gallon or two with plenty of RV antifreeze to prevent freezing. It gets cold in SD in winter.
Oh and we use angel soft TP. If you are not sure do the quart jar test on your paper.