How can I tell if my Aux Battery will charge?  

My Weekender came with an ancient dead Aux Battery (at least I assume it was dead - it was low on battery fluid in several cells).  I'm heading off to Mammoth Lakes in a week for a week.  I have a basic inverter and an extra regular battery I thought I'd slap in for the trip so the kids could have fans or use computers.  How can I tell if that battery will charge?  It's the stock system.

I'm pretty good with mechanics, but i'm clueless with all the electrical stuff - I understand basic circuits but thats about all.

Thanks in advance!

Dan
Los Angeles, CA
'85 Westy Weekender