It was a long effort but I would like to share my experience that it may help someone.
My black tank rinse stopped working on my Rockwood 2608 BS
First step - Check the back flow preventer. I removed the backflow device and checked the line from hose inlet to the backflow with compressed air. Good flow. With that I temporarily replaced the backflow with a strait piece of pipe.
Second step fill black tank with clean water and try compressed air after the water set for a week. Only a very small passage of air was observed (pressure at 60 lbs).
3rd step fill the hose leading to the sprayer in the tank with various solutions to dissolve the clog and use air pressure to force them to the tank. (Only solutions that are considered safe). Still no luck
4th step fill tank with water and "Unique RV Digest-it" (which was highly recommended on forums) and move the camper around a while letting the water soak for a while. Still clogged.
Lastly I continued using the "Digest-it" while camping and after my last trip of 2400 miles the clog finally disolved. Total time 7 months.
I hope this can help others with the same issue.