Wait a second. Lots of folks have the same converter -- very ordinary unit. Lots of folks have bought AGMs. Has anyone else reported this bizarre leakage under those circumstances? Not that I know of.
Bottom line -- something is wrong with that battery. This is not user error -- I happen to know that the OP is very careful and by-the-book. She should wear rubber gloves, blot up that gunk, and take the battery back to Costco.
And if AGM batteries can't handle being hooked up to a stock WFCO, the world should know about it so that we can avoid buying AGMs, if they are so difficult to maintain.