2002 Dodge Van losing power

I have a 2002 Dodge Ram 1500 Conversin Van, we replaced the battery and the alternator twice now, and still the check gauges light is coming on, and the battery gauge will drop all the way down while driving eventually causing the van to shut down. This does not happen while idling in the driveway. Don't know what to do, disconnected stereo, tv, and interior lights, replaced alternator after it was tested and the battery... what else could it be??