Avoid using products of the 'spray on leave 30 secs, wash off' variety. These usuaully have an acidic base, and they will penetrate the lacquer coating of the wheels, leading to unsightly blisters. A good car shampoo such as Meguires is all that is required-note that the better ones do not strip off any wax finish you may have applied. If you get little black specks stuck to the rim, you can get these off with an automotive clay bar, but for caravan use, you can even use blue tak with a suitable lubricant, though it doesn't last long...