2000 Silverado - Crazy Warning Lights

My dad has a 2000 Chevy Silverado half-ton with a 4.3 L V-6 auto.

Lately, it's been showing a lot of bizarre warnings. After filling up with gas, the fuel gauge goes to Full like normal but within a few minutes it will drops back down to empty and shows a "low fuel" warning. Sometimes this warning is also accompanied by a "Check Engine" or "Check Coolant" light.

That's really all I know. Any suggestions?

as far as the fuel gauge, i heard that the fuel sending units in these trucks go bad. it doesnt know how much gas there is. i think its like 150 for a new one. but as far as the other warning lights go i have no idea