Tonight I have to idle engine all night how many gallons will I. Use?

Why would you need to idle truck all night? Cold weather?

It will burn about a gallon and a half an hour on base idle.
On high idle it will burn just over two gallons an hour.
Unless its down below zero I wouldn't leave it run all night. On base idle it will sort up badly. Would take a while to clean out with a heavy load.

from what i've always read, it'll only burn just under a gallon of fuel for every three hours idling at low rpm.

http://www.dieselplace.com/forum/showthread.php?t=345542