Hi Chefdave,
I also use 75-90 in the rear end, and Valvoline Synthetic in the auto trans. I run 5w-30 Mobil 1 in the engine. Have not tried turning up the timing, let us know if you get a ping or overheating, and if it improves your power. I removed the roof A/C for less aero drag. A while back I recently found a Master's thesis online that I have found very informative on the aerodynamics of big rigs.
http://publications.lib.chalmers.se/...ext/133659.pdf
It looks at numerous aero devices and analyzes them with fluid dynamics and wind tunnel testing. After reading it I fabricated side skirts and plexiglass fender skirts for my Itasca that seem to be very helpful, although I don't have numbers. I have gotten my average mileage up well into the 18's now, often besting 20, with an occasional foray into the mid-twenties. I think that figuring out the transmission (the MPGuino is VERY helpful for this) helps with MPG as much as anything. The quicker you can get into the highest gear the better, and keeping it there as much as possible is important too. If you're into the 16's you've probably already figured this out. I find that accelerating briskly through each gear (letting off to force a shift to each higher gear as early as possible) and getting it up to cruising speed and into O/D lockup seems to be a good way to do it. On downhill runs, if you can get clear off the gas pedal at speed the fuel cutoff kicks in and shuts off the fuel flow and the instantaneous mileage goes to infinity. You can watch the average mileage climb during this time; it really helps. At a certain point as you slow, the cutoff disengages, If you plan to continue slowing you can click off the O/D, the revs will climb and the shutoff kicks back in. Great for pulling off the freeway. Mainly, the slower you can go at cruise (that allows the use of the highest gear in lockup) the better your mileage will be, all other things being equal.
Good ecomodding, and let us know how it's working!
Beaver