Some fenders just can't be cleaned... like mine! However, the best
results I've seen come from a couple of things:

Starbrite makes a shore power cord cleaner that works very well, but is
expensive. Another good combination is Simple Green and Barkeeper's
Friend scrubbing powder. Spray the surface, sprinkle the powder, and
scrub away with a Scotchbrite pad. This works for just about any vinyl,
including seats.