I have a 99 Isuzu Rodeo and my battery wont hold a charge. whats wrong?

I put new battery cables on yesterday and jumped it off and it crank everytime I went and tried it. This morning it was dead. So I put in a new battery. It crank right up. I drove it to the auto parts about 2 miles to have them test the alternator and it had already drained the battery so they were un able to test it. Now it wont even hold a charge so I had to leave it there and walk home. Do you think it is the alternator. I just hate to spend that kind of money and it not be the problem. If anyone has any ideas let me know