Well my option is I think it's the GP module but I wanted to see what you guys thought..

28-30 deg cold start fires up bat gauge sits at 14amps ....Driving truck at a steady cruise voltage starts to drop off from 14-10 and goes down.. If I goose it the volts climb back up then a while later they start to drop off again...I have replaced the alternator with a new one but still doing it.. Any thoughts

 

I am Not a Chevy 6.5 guru but you might check your fuel heater by disconnecting it / drive it and see if that changes anything.
Sounds like a large load is being applied to the charging system by a consumer. Glow plugs should be off once the engine fires up and not sure if a glow plug module would be causing
the problem.

fuel-heater-replacement.pdf

Wouldn't be the first time a parts store sold a junk alternator either .

 

Checked grounds ,cleaned battery cables and replaced the GP module seems to have done it ..165k original GP module think it was time...

 

Nice good to hear!!

 

Check also the ECT sensor the GP control is "dumb" it relies on ECT/PCM to tell it that it is cold out, once you are out of start mode your glow should be out of the loop, next time you see this occurring maybe put a meter on the output leg of the glow controller to see if it is in fact sending voltage out to the glows after the start sequence.

 

Following this thread. We replaced the alternator. Sounds like a bad regulator.