my van is; 2002 savanah,1 ton,(3500) 8.1 v8 gas, single wheel,

i cant find mpg rating and i looked online 45 mins...plz someone tell me what can i expect.ive never owned that big a v8 in a van. it will be used as a plumbing van. the owners manual is gone, so i have no idea what its rated at. thanks.