True story: Man had your problem, turns out the RV body builder, when they cut the hole in the floor to install the Toilet, they dropped the wood (2 half moons) into the black tank, where they eventually slid over the outlet and blocked it.. The owner managed to remove them (Do not ask how as I do not know) and sealed them in baggies (double bagged in fact) and returned them to the CEO of the RV manufacturer company, or so he said. (And again I do not know how).
Suggestion.. Start by closing the valve.. Get a short (5") clear extension and install it on the outlet. (NOTE: if you get a flush king with the clear part you may not need that) now Assuming you have a gallon or two of room in the tank.> Open the existing valve only (keep flush king closed) force 1-2 gallons of water in via the vacuum breaker/back flow preventer and then open the 2nd (Added flush king) valve, see what happens.
if the problem is something blocking the outlet, this may break or move it.
This is not a normal "Rinse out the tank" issue so the video does not apply.
Option two (The coat hanger method) makes one mel of a hess as Mr. Spooner might say.