Black tank not draining!

My flush system flushes the tank but only clear water is draining. It is apparently plugged up higher. When i try to flush with the pedal, it appears to be full.. i put in a blue pack and tried to use the flush system, only clear water goes out the sewer hose , clear plastic. How to clear it if the plug is higher up from the tank? I don’t have a sewer snake but is that what I should try?
Thanks