2007 Silverado 1500 z71 vortex max has stalled on me twice in the last two years while towing my 4000 lb travel trailer. First time the alternator was replaced. Most recently, tests could find nothing wrong with truck. Trying to narrow down issue. Any ideas? Btw, it does not restart right after it stalls. Both times it happened was at the end of trip - almost - coming off exit ramp of hiway.