It doesn't. Idling takes about half a gallon per hour at most.I run it till the light turns on, then fill up till the pump shuts off, (20gallons or so). I sometimes leave my truck in idle while I run into 711 to get coffee and breakfast sandwich for my dad and i, which doesn't seem too long ( 5 mins maybe). Didn't realize idle can take that much gas.
The light turns on when you have about 3 gallons left. The tank is 21.1 gallons. If you're truly getting 180 miles on 20 gallons, that's 9 miles per gallon. Nine miles per gallon while feathering the throttle at 2500rpm as you mentioned clearly points to some other issue. But the only way to definitively know is if you start logging data instead of approximations.