7.3 (towing) is "fantastic"? Well...depends. How heavy is the "toad"? A full water tank has a negative effect (I usually have it filled 1/4 for the toilet, if needed). Propane...should keep it topped off especially during the coler months, but I don't think that has any real affect on gas milage, like the full water tank does. Are your grey/black tanks empty? If not, why not? Tire inflation? Too little...that takes away, even if just abit. Speed: More than 60-65, (on cruise control)...just watch that gas gauge dump. I run around 62 (tach reads 2500 rpm). Any slower on the Interstates, and you're looking for problems. Stop and go...traffic...eats it up. Can't help that....
Just my opinions...FWIW.