Alternator doesn't start charging when I first start truck

Regulator/zenor diode on its way out. Mights as well get another alternator with a pulley to carry with you. Might last a week might last a year or more. I had one like that that lasted forever. It would droop when l first put all the lights on. I woud turn the lights off for a second and it would be fine for the rest of the day. Did that like clockwork for at least a year.

 

Check your battery cables. Don't just look at them. Remove each lug. I will bet at least one of the ends is broken off.

 

Yep, check all cables, ground and power connections. Do voltage drop tests between alt and batteries and cab.
Now, the only thing with your truck, the volt gauge is feed data from the ECM. What ever the ECM gets, thats what the gauge will read. So the cab and batteries may be just fine, but the ECM power harness is faulty somewhere. Start the truck and check voltage at batteries and alt output to make sure.