Hi Marshall,
I haven't logged it too closely in my ownership, but IIRC about 11-12mpg.
Maintenance costs are the gem of the 6.8L V10; SUPER reliable and MUCH lower maintenance/service costs than any of the diesels(7.3L or 6.0L PowerStrokeDiesel), whisper quiet too! Many members have 100,000-300,000 on their V10's and still running very strong. Basically change the oil/filter every 5k, air filter every 30k, 2yr brake fluid flush, that's it!
The 5.4L V8 is the only other very reliable choice, but nowhere near the power/torque/drivability of the V10 (if you were to 4x4 it, you'd definitely need to re-gear the heck out of the V8)...plus the 5.4L V8 is a 4spd(4R100/4R75W/4R75E in van's) vs the V10's 5spd(5R110); 5spd is so much nicer shifting, easier to drive, with an overdrive on/off(tow/haul mode of sorts) button on the shifter selector.
LMK if you have further questions,
Jeremy