Before I removed the clutch fan (and replaced it with electric) and added the grill block, mine never got beyond the lowest part of the "L" of the L------H temperature gauge either.
You asked me the same question before. I still say driving slower made the single biggest difference.
Looking back at my fuel mileage log on EM over the past 2 years, no one thing stands out as making a hugely dramatic difference.
1983 Ford F-250 Gas Mileage (Big Orange Work Truck) - EcoModder.com
Its been gradual incremental improvements from each step, with the exception of my first tank after reading the article on Wayne Gerdes in which I first learned of the concept of "hypermiling".
In terms of combinations of things, did removing all the engine belts (fan, alternator, power steering, and vacuum pump) help more or less than the aero changes (grill block, underbelly panels, wheel well skirts, and tilted tonneau)? I have no idea, as they were all incorporated together haphazardly as my ecomodding hobby grew.
All I know is I started out getting the same mileage as you are getting now, and 2 years later I am averaging just under 30mpg for about a 4 month span.