I have used a roof treatment in the past but did not see any good results by using it. Now I do nothing but clean it a couple of times a year.
Our PP has the built in black tank flush but I also have a Flush King I connect to the sewer drain. It is clear so you can tell how clean you are getting the tank when you flush it. You should see your black tank level rise when using the flush. Just keep an eye on the tank level to make sure you don't over fill and have it going out the roof vent. Don't ask how I know this......
We have our unglazed tile on the rack above the burner.