Had that problem once before. Bought a flush king to backfeed the outlet and that did the trick. whatever the clog was it pushed it back in to the tank ( or broke it up ) and allowed he tank to drain. just be careful you don’t blow anything out of the toilet.
Reading this it's confusing? How do you know the tank is empty? If the bowl is full and the tank empty then the clog is in the pipe from commode to tank. Any kids who may have stuffed something down? Do you have a plunger that can put pressure on the clog?
Not a good thing the day before Thanksgiving if you are camping.