Slimey grimey by star brite is the best stuff on the market. Comes in powder. Throw it in a spray bottle hit the hull before splashing and you are done. Best part it doesn't hurt the trailer. Took me 5 mins to apply yesterday and hull looks great!!

Nautical Ease was the best stuff I have used but have no used it in about 4 years so possible their formula has changed.

Walmart toilet bowl cleaner. Look for 9% hyydrachloric acid! Buy a cheap mop. Do small areas and rinse liberally.