It is a mild abrasive, that is what is used in fiberglass refinishing kits, it is just super inexpensive. The finer grit you use on the scrubbie, the more the Bar Keepers Friend is effective on black streaks, etc. White scrubbies from 3m are finer grit than the common green ones, but they are harder to find.
Scrubbing with BKF removes chalking on the surface. If your RV is not old enough to have chalking or black streaks, it probably doesn't need the Bar Keeper's Friend step and you can just wash it well, a couple of times is usually good insurance, and then use the Red Max finish for about 4 coats.