Don-
That sort of corrosion is not unusual for these coaches. I had to rebuild the tray on our old Pace Arrow once; the worst rust was under the two 6 volt batteries. Our rig while in storage by the house gets almost exclusively solar charge maintenance, and the batteries don't overcharge. Our controller is on the bay wall just in front of the curbside rear tires, but ours is an '06 Monty. You've got a lot of batteries cookin' away even when the charge rate is correct. The gassing is there regardless, "boiling" or not. Upon hitting cooler air they sink and condense on cool surfaces arguably like acidic rain. Even small amounts of condensate can deteriorate metals with which it readily reacts. A good coat of resistant paint helps as does regular cleaning, but few of us remove all the batteries often enough to do that as it should be done. And of course be certain that those 4 deep-cycle 6 volt batteries have distilled water up to about 3/8" below the bottom of the fill ring, and be sure to use eye protection when you do.
On my coach the vulnerable parts are the buckles that are on the battery straps - even if chrome coated metal, the acid eats right through in short order, and if plastic, it melts. The straps are bracketed to the tray and those brackets seem to be prime fodder for the trace acids, despite my regular application of anticorrosion paints and Corrosion-X. Invisible condensate seems to just naturally "gravitate" over the sides of the battery banks and down, and everything at the bottom or in between is in harm's way. The wire connections at the top, which on a car is the common target for sulfur "bloom" and corrosion, are very rarely corroded on our coach; possibly because they are more obvious and easier to clean and keep treated. I even tried slipping those red and green felt terminal rings over the strap just up against the buckles and the brackets bolted to the tray, but that doesn't stop it.
Next time I get new batteries, I'll likely clean everything and add a coat of POR-15 and topcoat, and include coating the buckles and any new brackets. POR-15 is pretty tough stuff and should help resist the deterioration. It certainly did on the Pace Arrow's tray rebuild - no rust at all, ever, after that.
Joel
