The 6.8L is about 25% larger displacement, and under best case in town light-duty driving does seem to use about 25% more fuel than the 5.4L.
However, once you start adding a load, the difference gets smaller as it takes fuel to make power. At some point, the 6.8L will use less fuel. Since you aren't towing, you won't ever run into that.
I average about 11-12 highway with my V10, however it has a huge ass grill guard, full roof rack, is a dually, and has 4.56 gears. My dads van is all stock and has the 5.4L with 3.55 gears and averages 16-17 highway. When my van was bone stock with 3.73 gears I averaged 14-15 highway. Sure was nice back then when I could go 500 miles on a tank of fuel
If you aren't towing, you definitely don't need the V10. You won't ever "wear" out the V8. Either motor can easily top 500,000 miles under hard commercial service, and owners who aren't driving as hard can double that. The V10 is paired with a much more robust transmission, but if you ever did have trouble with it, it will cost more to repair than the 4-speed behind the V8.
I haven't seen any substantial difference in maintenance costs between the two, other than the extra pair of spark plugs and boots you have to buy every 100,000 miles.