Hydrogen sulfide gas from the vents readily attracts to bare metal and even the slightest nano-gap in painted parts nearby. It can settle on the battery top and migrate down strapping to buckles and fasteners, and down the battery side to the tray. I mix baking soda with water, apply it with squirt bottle or brush to the batteries and attendant parts, let it fizz, wire brush any heavy sulfur bloom, then gently rinse. Just don't splash too much or get soda in the deep cycles' vents - soda and acid don't mix well inside, obviously. A full air dry, then use battery connection paint wherever I can get it. I even use Corrosion-X as a backup.
But certain susceptible areas still rust, sulfur bloom under paint, and deteriorate. Every few years I reckon a complete battery removal and tray renovation is necessary. On my last coach, I finally did that and painted every steel surface with POR15 paint; no more problems - that stuff is tough. I've yet to redo the Monterey's tray, but will maintain as best I can until battery replacement time, then breakout the POR15 while the tray is empty.
Joel