More than likely the tank is full due to the bubbling when you were able to flush but verify that by opening flush valve on toilet and using a long piece of 2x2 and push it to the bottom of the tank. If there is no blockage in the pedestal or floor flange then the tank dump valve is either not opening or is blocked by tissue or something. Verify the valve feels like the blade is being pulled back by evidence or a little liquid weeping out the termination connector. Be mindful that if it is blocked, it could release at any moment so, be prepared to close it in a hurry. If you feel it is blocked, use a back flow type wash out fitting on the termination bonnet fitting and connected to a sewer hose. Try forcing water pressure back into the tank in bursts. Hopefully you can break it loose and get at least some draining. If you can get it drained some but not completely, you can contact a septic tank cleaning company and you can try getting them to pull out the blockage with their pump out truck. Many times I have had success with this but if not, then you will want a RV repair service do the repairs. Good luck, stay safe and keep us posted.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7