2007 dodge ram 1500 battery gauge fluctuated and check gauges light on

I don't know a lot when it comes to vehicles but i
have been trying to learn so I am putting this out
there hoping someone might help....the lights on
my truck had been left on for about 6 hours, I
went started it drove about 2 miles parked,  
noticed that the lights were on and that the
battery was on the low side not really low tho.....i
turned my lights off and started it again it fired
right up but the battery indicator kept moving
from half to the last bar before red. Then the
check gauges light came on. I stopped at
O'Reillys and had them hook up to see what the
light was for they said I needed a new
alternator...bought and installed, didn't help...put
new battery in it seems to be stable running at
11.3v-12.0v. but the check gauges light is still on.
I have checked fluids and so on but everything
seems to be fine. Is it my computer..belts...i do
not know any thoughts??? Please