First off, welcome.
Now to your question:
You figure typical tunner is going to run about $500 and you might gain 2 mpg.
So say you have th following.
10 MPG over 1000 miles at a price of $2.60 per gallon you spend $260.
12 MPG over 1000 miles at a price of $2.60 per gallon you spend ~$217.
So for every 1000 miles you save $43. @ about 11,630 you break even on the cost.
Now if you add a tuner you will probably want a free breathing exhaust to go along with it to hel keep the Exhaust temp down. So add another ~$500. So now you are looking at 23,000miles to break even.
Auxilary transmission will run you over $3,000. Lubricants i doubt would gain you much. I've heard some claim better milage with Stanadyne additive, but have never tested it myself.
Propane when used as a mileage boost works, but I have never sat down and figured out the cost. MSD/Superchips makes a propane system for our 6.0s. I'm not sure how long the propane lasts, so i don't know how much propane you are burning per mile. I've heard as high as 25MPG with a propane system on an empty truck, but not knowing how often you have to buy propane I can't say how much you are actually saving.
I'd say your mileage is decent. The Van Powerstroke has a little less power as I think it is detuned to keep the heat down. I get anywhere between 8 and 11.5 towing my 13,300lbs 5th wheel.
Hope that helps some.