OP,
If you follow these forums long enough, you'll see many repeat posters and many are truly honest replies. In saying that, I reiterate my point that most Ram's and GM's get better mpg's that do Ford's. If you plan on towing anything north of 15k (RV) it is strongly encouraged that you go the DRW route for safety and if you wish to add an aux tank in bed of truck seeing you'll have more capacity.
So based on the replies you've seen. most folks are getting between 8.5-11 towing and 12-20 empty. Gearing, tranny's, etc all faster into it as you know. My FIL (RIP) asked me many years ago when I had my 6.0 PSD and fuel was going up then about regretting getting a diesel and my reply to him was: "Nope, got it for power". I will say tho, the 6.0 I had did get decent mpg's.