I haven't noticed a decrease. A 1-2 decrease would go unnoticed by me. These vehicles require gasoline for them to move. I'm dang sure not walking to work.
I drive the same distances most every day, but on some days, I may be pulling a trailer, boat, etc... Can't compare mileages that way. I have yet to polish off a a full talk of fuel by driving my Jeep with nothing in tow if that tells you anything. However, this week I'm on day #4 after filling up and have yet to hook anything to the hitch.