Hi Patrick,
Generally speaking, a drum doesn’t usually need to be cleaned and the natural brown patina of the drum actually enhances the flavor of the coffee. But if you should need to clean it you can use soap and water and a brush then hose it off. You can also let it sit in a soapy 5gal bucket for the inside. If your drum is sooty, black, I would have a look at the condition of your burners and your grill. Also, it could be sootier if you dual-purpose your grill. (Meat/coffee). You might be able to use oven cleaner though I’ve never tried it.
-Shane