you mentioned the volts were steadily dropping.. was that just a rough observation based on lights and stuff?
I would go to the parts store and pick up an voltage gauge for about $20 or less and install it. for now just plug it into a power tap in the fuse block and ground it well... if you don't want any gauges installed permanently, just zip tie it to something
the battery charger you can pick up just about anywhere with an automotive section. WalMart, K-mart, Target, or your favorite parts store. the smaller ones will be cheaper.. I'd recommended the fully automatic maintainance charger from Schumacher, it's 10A, and it will turn off when the battery is full and will cycle on and off as needed to keep it full in storage.. I paid about $40 for a charger like this a few years ago (from AutoZone).. and I love it. It even survived being under the hood of my car when my drunken neighbor tried to shove my car through my kitchen with his van.
it was thrown right out from under the hood, the cord ripped right out of the back of it.. I put the cord back in it.. and it still works great
This is the newer version of my charger.. mine doesn't have the trickle and "Desulfation" modes. but the thing is it has automatic shutoff, and if you leave it on the battery in storage, as the battery starts to loose its charge, the charger will turn back on and charge it up again.