2001 Dodge Dakota Battery being drained while driving

Just bought this truck, put a brand new battery in it. When I start it, the battery gauge is on but within a few seconds the gauge starts going down quickly (but doesn't 'switch' off), the check gauges light comes on. If I turn the truck off and turn the key back to the on position it gauge will come back on each time, but each time it will go off within a few seconds. I took the alternator in and had it checked. I've checked the battery connections and grounds. Checked the fuses. When I was checking the fuses I had the car running for less than ten minutes and all the gauges and inside lights went out, no headlights or radio and the engine was barely running. I shut it off and tried to restart it and nothing. I unhooked the battery and checked under the battery tray for possible corrosion on the harness underneath it. Didnt find anything. Is the most likely cause a dead ground sucking the power out, or is it the PCM? PLEASE help.