Most seem to think that the slowest speed that keeps it in the highest gear will provide the best economy. For me on a day with no headwind and on the level, I can do that if I set the cruise control at 61. I've never calculated the milage by hand but the Aladdin says my average for 120,000 miles is 8.1. My usual habit is to set the cruise control at 63. When traffic allows, the scenery is worth it and I'm feeling extra cheap, I seem to be able to get as high as 9.0 in fifth gear at 55 mph. I have an 8.3 ISC 350 Cummins and our coach weighs about 33,000 pounds.
In 2006 Cat wrote an 18 page booklet called "Understanding Coach/RV Performance". I no longer have the book but do have it as a .pdf file. It is full of charts, graphs and facts relating to fuel economy. You may be able to google for a copy. If not, I can email mine to anyone that wants one using the email address in the BAC Directory.