Howdy Hugh, I drive an Excursion also, but mine is a gasser V-10. We use it as our tow rig for our large travel trailer and with a couple of mods I have increased it's fuel mileage by about 28% while towing. It used to get 7 MPG while pulling the 11k TT with the stock 3.73 gears, swapping up to 4.88 gears (and going to 35" tires at the same time for an effective ratio of 4.39) and installing headers bumped the towing mileage up to 9 MPG. 9 MPG is about the lowest MPG you will see anyone on this forum bragging about but a 28% increase is a 28% increase!
On your PSD EX I would look to loose the cowcatcher, lower the suspension (a couple different ways to handle your 2X4 frontend drop), keep the load range E tires pumped up to 80PSI (I hope you have E's since you talked about towing 10k lbs), go to a narrower tire than the factory 265s and look for a slightly taller rear tire to give a lower numerical effective gear ratio.
Don't rely too much on the factory mileage meter, they are commonly refered to as Lie-O-Meters, do the math by hand for accurate results. I would also recommend visiting a few Ford truck forums and read up on 7.3 info about boosting MPGs, it gets discussed a lot. (I'm active on Ford-trucks.com, plenty of good PSD info and troubleshooting tips and a great Excursion forum that would be interested in your goals and mods!)
Here's a shot of our EXcursion about to earn it's keep.
