Great test, and results you got aren't too surprising. :)
In the interest of doing a fair comparison, you should have used a Flush King or Hydroflush with the 3" hose setup as well, and used it to do a flush afterward like you did with the Sewer Solution.
I'll bet if you did that, you'd find that the only real difference will be that the sides of the tank will be a little bit cleaner after using the sewer solution. Bottom, amount of solids left, will be about the same, then, after a flush is done.
Others have made a very good point, too - Very, very few RVs are plumbed so that your flush would have a straight shot up into the black tank. Any chance you can fit an elbow in there and try it? That might be more realistic.
