Phil,
Fuel mileage is well and good, but another reason to consider is slowing down is for safety.
I had a front tire BLOW OUT @ 65mph on I-25 in Santa Fe, NM. The coach tracted straight and I slowed down gradually and pulled over onto the shoulder. It doesn't take long for the tire to disentergrate. No damage occured and we were on our way in a little over an hour with a new tire from the local truck service. This little incident convinced me to slow down. I used to drive 75 + !!! No more, now it's 60 & 65. I get better mileage and my family is much safer, plus it is easier for guests to walk around in the coach at slower speeds.
Just my opinion. Enjoy your coach.
Tuga Gaidry