I have used "Reliable super spray cleaner" made by Reliable Products Oneida, NY. Sprayed on, then soft bristle brush, then warm water, etc. takes a lot off, but it never looks as white as new. This is an alkaline solution. I notice the previous poster uses vinegar, which is acid. Using these two, one after the other, might well be the best answer.
2006 GMC 2500HD 6.6 diesel
2003 Outfitter Apex 9.5