My Son-in-law had the same issue last year with his 5th wheel.
Only a trickle was coming out.
We added an liquid digester (from an RV retailer), let it sit and work for a while, then took it out for a drive to let it slosh around.
After about 30 minute we tried the dump again and got a complete empty.
We never really did find the cause, but expected it was a grease build up the had come loose.
I'm sure using a heavy duty clean should dissolve any clogs and then using the Blue Dawn will keep things clean moving forward.
Again, good luck with it.
2014 Fleetwood Southwind 32VS 🇨🇦