1981 Sierra Half Ton Fuel issues
 

I got my Sierra yesterday and it is my first vehicle so these questions are probably going to seem odd.

It has the twin gas tanks and no matter what tank I choose the needle is pointing at 3 o'clock when thinking of the dial as a clock. Expect that the empty to full spans about 10:30-1:30. When I turn the truck off and let it sit it drops down to about 2 o'clock. But then when I turn it on the needles flies right back to 3 o'clock. Are the tanks just that full or is something wrong?

Also, how do I actually pop the fuel flap? Do I just get in there and manually open it? I saw no button anywhere on the dash.

Any help is appreciated.