I think your mileage increase from 5.9 to 8.9 was due to the last gas fill not being completely full. Your ave would be 7.4 which is consistent with you other readings. I have noticed that all the time when refueling. Having said that I get better gas mileage at 55 than 65. What does your MH weigh? I have averaged 8.7 mpg over 80k miles driving between 55-62mph, 15k lbs 454 tbi. I think your mpg would improve by not using cruse control and keeping it in high gear by backing off the throttle when climbing a slight hill.