Black water Full valve stuck closed

Sounds like your clog might be at the toilet and the tank is actually empty. I would hook up a clear fitting and then the drain hose to a dump valve. Then use the tongue jack or level blocks tilt the trailer so the valve is the low spot. Now open the valve and see what comes out. If it only trickles and stops leave the valve open send someone up on the roof with about a 1/2 gallon of water. Have them dump the water down the tank vent pipe while you watch the fitting. If the 1/2 gallon flows out of the valve then you know your tank is empty and you have a clog at the base of the toilet. If nothing comes out you have a valve clog and hopefully the addition of only 1/2 gallon of water will not cause a catastrophe.