Of course, these RV disasters always seem to happen on a weekend, or just before, when service help is minimal.
Backflushing is a risk of black water flow into the rig, but the toilet valve (footpedal) would have to be open to allow this unless something breaks.
Maybe the least risky first thing to try might be a toilet snake that would help open any blockages immediately below the commode. You might want to turn off your fresh water so the toilet flush with the foot valve open wouldn't add to the fluid load.
Just as a troubleshooting aid, try to drain some water out from your shower or kitchen sink to make sure the end drain piping is open.
I hope it is something as simple (to find - not to fix) as the cable to the black tank valve is disconnected. Do you feel the normal resistance when you try to open/close the black tank valve handle, like the other valve handles feel? BTW if your black tank valve seems not to operate with the handle in the UDC, and you don't want to do the big fix on the road, You should have an external sewer drain valve if you opt to leave the black tank valve open.
Here is a link to a You Tube search on unclogging RV toilets:
https://www.youtube.com/results?search_query=toilet+drain+snake+rv
Here is a link to using an external valve on your sewer drain system: