1992 GMC Vandura 2500 won't start

Hello,

My GMC won't start.
I hear the click, but then the engine won't crank.

At first I assumed a dead battery, and got a jump, and then it was fine for a while, but after a time of not starting it for a bit (I don't drive it daily), again it went dead. I got another jump, used it on and off for a few days and it was fine, then left it for two weeks, and it was dead again. I checked the battery with my multimeter and it had a full charge of 12 volts. So I figured maybe it was the starter or the cables. I looked at the cables and they looked good by visual inspection... a little bit of corrosion on the outside of the contact on the battery side, but the contact itself looks pretty clean. I tried hitting the starter a bit with a hammer while turning the ignition, and it was trying to start, but not starting. I then remembered that the previous time that getting a jump made it work, so I tried that, and it worked. After driving for a bit, full charge again, and starts up again after turning it off, just like last time. But I assume, like last time, it will go dead once again if I leave it for a couple days.

I'm not sure what this means... is it a battery issue? The battery had 12 volts, though on the dash it showed nearly dead. Is it a starter issue? Why would getting a jump make it work if that's the case?

Any help much appreciated!

Thanks,
Mike