Thanks for Choosing Just Answer.
What kind of RV is This? IS the Black tank full?
SO it appears the tank IS not full. Can you tell me the manufacturer of the toilet and model, sometimes this is right on the toilet, this would be helpful, it sounds as though the valve in the toilet is not opening and there are some different setups.
OK so you are saying the valve opens up in the toilet and it is not going down at all? OR the toilet is not filling with water?? Do you feel your tank indicators work?
There is a chance the piping from the toilet to the tank is clogged. The valve opening should allow the water to drain. You can try to get some liquid soap, pour some in and get a large amount of ice to push the blockage through, the bad thing is you may have to take some water out before putting the ice in or it will make a mess. The other option is to try to plunge it out. If this is an electric toilet the power will have to be disconnected while the valve is in the open position. IF this is not a electric toilet,you can shut the water pump off and step on the pedal and try to plunge. The distance from the toilet to the tank should not be that far so the ice may do the trick.
The only other idea I have is to hook up a garden hose and with the valve open put it down in there and turn it on. I must tell you this is pretty odd. By any chance do you have two bathrooms? The reason being sometimes a macerator pump is installed that helps chew things up, normally they are used in a dual bathroom set up. I would verify in your plumbing bay you can see the plumbing from the toilet into the tank. The only other issue would be if your tank really is full, and your indicator was giving a false reading..... You Could try overnight to let the soap and ice work but you will need to leave the valve open. Please let me know what happens, things are hard to fix without seeing them in person, Oh one other thing Monaco coach who makes holiday rambler has a excellent technical support line, you may want to try to see what they think. I am not sure if they are available over the weekend, if you do call make sure to have your Vin and unit number ready to give them.