before spending money on an alternator try this.

Its called a big 3 upgrade
Alt to battery positive
battery to block neg
block to frame neg

upgrade those 3 wires to 4awg. It will make your charging MUCH more efficient and may fix the problem. If that doesnt do it, then upgrade the battery to a gel cell like an optima. Those types of batteries hold more juice and charge and discharge faster than a stndard lead acid battery. If those 2 fail and its still having problems, then its time for an alt, but even if you know your going to need a better alt, to get th most out of it, you should do the big 3 and battery upgrades first or at the same time.