So here lately I have had to jump my truck off. It's got a fairly new batt. I can charge it but when the truck sits for about 3 days it goes dead. When I drive it the charge shows about a 1/4 less than half at idle with out air on. And 1/2 at high rpms. I can tell the light dim at idle too should i change alternator or what do I need to check

 

Could be the voltage regulator or the alternator

 

Or you have a short in the system some where. If the battery is going dead after a few days I'd want to know why.

 

Check the battery connections and the ground. Make sure its ok and not corroded.
I would also check , at nite time....to make sure there isnt a light staying on, glove box, ash tray, dome lite, etc.

You can pull off the alt and get it checked to see if its ok too. No need in buying one if its not bad. Next would be the voltage regulator.

 

We have lights in our ash trays? I'm not trying to be a smart "butt" here or nothing just, out of the 4 of these trucks I've had/have I've never seen one there! Now I'm gonna have to go out and check, and I was all ready for bed. LOL
As to the OP, I'd check for a drain like others have said. Then go get the Generator checked for output. Does it have a slow crank if you try to restart it right after shut down? A bad cell in the battery can cause all of this too! Good luck. A parasitic drain can be a PIA to find. We have the good fortune not to have many electrical componets on our truck though!