Did mine today. Used Dawn Dish soap. Went over each area 3 times. Used the same auto brush I use to clean the RV. You should be able to tell what parts you missed. I think I still missed spots but I could tell that it was cleaner than when \ I started.
I did note that there were these little grains of white, like sand on the ground when I was done. I think that was the part of the membrane that washed off. I think it was the equivalent to exfoliating it. Of course, mine is 6 years old, not as old as yours..
The neighbors loved seeing me on top of the RV and a bunch of them stopped by to see what I was doing.
Home: El Paso Texas
Jayco Grayhawk 31FS 2010