It sounds like the check valve that is inline in your black water flush line is broken. Same happened to me in my 2008 25fb Classic. I decided to replace it with elbow and plumbers tape on the thread. The valve is located behind the shower stall but you can get to it via the water pump access panel in the hall closet next to the shower. The PEX is flexible enough to bring the failed fitting into the area to work on. The Pex is a vertical pipe with the valve at the top and then makes a turn to another vertical pipe down to the tank. I purchased the gray elbow from the plumbing section of an RV parts store.
The check valve is a clear plastic valve. Probably water left in the valve and it froze during the winter. Previous owner of my trailer probably never blew out the line with air.
Once you get it fixed I run water into the black tank until its near the toilet. I set the monitor panel so it reads the black tank constantly and when the indicator shows 3/4 I turn off the water. I open the shades of the window across from the kitchen so I can see the progression. I'd verify visually until you verify the monitor panel is reading somewhat accurate.
2008 Classic 25fb "Silver Mistress"
2015 Ram 2500 6.7L Cummins. Crew Cab, 4x4, Silver
2010 Tundra 5.7L, Double Cab, 2wd, White