I just scrub the dickens out of them. They get cleaner but after a couple years there is only so much you can do with them (mildew stains won't always come out) but spend the bucks for new if you really want them to sparkle. One set on a smaller RV ended up stained through from the tires but that was probably as much from using the now dreaded Armor All. Probably the much loved .303 will do the same as well but without the tire deterioration.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')