I understand that you want to have the flusher work, but on the past two 5ers have found that they really aren't much good with only a single point metered spray. I've found on ours that the black tank flows pretty slowly when being dumped which of course will cause solids to build. Camco has a flusher elbow that fastens to the dump outlet and does a great job back flushing that tank, much better then that puny spray from the built in system. I have been able to flush a lot more ... stuff then before. I also use the built in flusher with the macerator and it seems to work even better. I probably haven't used the factory flusher more then a couple times in the past year or two. I also have a home made flusher that I shove down the toilet but it's much less effective then the Camco gadget:
https://www.amazon.com/Camco-39062-F.../dp/B000BUQOAE