Miles per gallon on a 99 7.3 auto

[l.a.w.1111]

Hi guys,
I need you to help settle something. I have a 99 F250 Extended cab 7.3/Auto/3:73/2WD. Everything on the truck is stock except it has Firestone lift bags. I'm pulling a 94 model 29" 5th wheel, I do not have the gross whight on the trialer.
The question is this: towing in the summer on flat ground with no wind @ 65mph, what kind of milage do you think I will get? I say 10, my buddy keeps insisting that it will be around 13-14. Right now I get almost 19 on running winter fuel on the flats at 65-70 empty.
Thanks in advance for the input.

[S.S construction]

You maybe get around 12 tops id say i got 2002 f-350 with 4.10s and i get 18 empty and 12 or so with 15,000lbs

[JJR]

Should be about 10-12. I pull a 10000lb fiver mid profile and get 9-11 mpg once in a while I will get 12 but that is rare. This is pulling at 65-70 mph.