Help Now! Im stuck.....Please help

[twenty2]

To make a long story short, yesterday when I was putting up Christmas lights up for 5 1/2 hours, I was listening to the truck radio with my key in the ON position. When I was done, I started up my truck and it was dead. I jumped the rig, and it started.

Problem now is when I drive the truck down the road, my truck dies! RPM's drop to zero causing the truck to shut off.

The truck starts right up, but I notice that my battery voltage seems just below 14 fluctuating.

When im at idol and i put on heater ON, my truck dies. You can notice the idol goto ZERO as soon as the heater kicks in.

Is it my battery or my alternator??? Please help..