I don't understand how a battery can leak bad enough to rot out the tray and hold down hardware but the truck's owner doesn't replace the battery????? Battery leaks acid, replace the damn battery.
Buy an AGM battery.

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The tray, like any other metal part, is subject to rust. Considering the truck is a 98 and it’s now 2020, it shouldn’t be surprising that there’s rust all over the place now. The bolt rusted out a couple of years ago. I don’t think it had anything to do with the battery as the battery is in good condition.