I can't speak for the Allegro for sure but on my Endeavor I have been having exactly the same problem. Yesterday I removed the flush system from the black tank (three screws and a water line mounted on the top of the black tank). The inside of the flush wand was full of what looked to be pulp from toilet paper and all the small spray holes were plugged. I cleaned out the pulp and drilled out the spray holes to 7/64 and re installed using clear silicone to seal the unit to the tank. The whole thing took less than 30 minutes. I can't tell you where your unit would be but mine was accessible through what everyone calls the hidden door in the side of the motorhome which opens by removing two screws rather than using a latch.
Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad