Lights working, but had to jump start. Not sure whats wrong?

[potbss]

All the electronic stuff was working but the truck didn't have enough juice to start. I'm not sure how long the batteries have been in there. my truck is an 2003 psd 7.3 77 kmi i bought about 3 months ago. I order two new interstate batteries. Another thing is that after I jump started it, it ran fine but after awhile of driving it began to loose electric, like radio, horn, and power locks windows. Truck still ran though.

Now i'm not sure if its the alternator or batteries. any suggestions should I still go ahead and replace the batteries then test alternator?

thanks