Truck loses power at higher speeds

Truck starts and runs somewhat okay and does not lose power at lower speeds around town. Took it out on the highway the other night and within a couple of miles approaching 100 km/hr all the lights started fluctuating and dimming. The battery gauge was jumping back and forth until all the gauges dropped, the truck lost power and coasted to the side of the road. I checked the battery and alternator connections and all were secure but the battery was dead and required a boost. I boosted the battery and the truck started right up. I did a load test on the battery last spring and it was under capacity. I checked the eye on the battery today and it shows green. The truck does idle quite low but it has never stalled. Could this be a battery and alternator problem? Should I buy a new battery before also replacing the alternator? Thanks for your help.