Was it shiny at some point or was it always a flat finish? It depends upon whether the diamond plate was/is polished or plated. Either way you can start with a good aluminum polish and a polishing wheel on a drill (or by hand). There are many products to polish aluminum but the easiest to find is made by Mothers - usually available at most any auto parts store, Wal-mart, etc.
94-Newmar Kountry Star 40-KSDP
Spartan/Cummins 8.3C-300HP/Allison 3060 WTEC-II/25yr RV Tech RVIA Certified/Onan-Cummins Certified