Hi Eric,
Fenwicks caravan cleaner used neat if the stain wont move has never failed for me, even better if the caravan has a coat of polish on it first, I use Autoglym and the stains come off easily with a bucket of water with Fenwicks applied with a washing brush. The other brands are no doubt very good but the first I bought was Fenwicks and had no reason to change.
Regards
spj.