My overdrive kicks in at about 60 MPH, so I travel about 65 on big interstates like HWY 10. I'm never in a hurry when I'm in my RV. When I see a vehicle approching me from behind, on a 2 lane road, I pull off the road as soon as possible. Sometimes this means driving down a dirt road until I can turn around and get back on the 2 laner. Do you want to conserve gas? Put a glass of water on your dining table. Now drive without spilling it.
Relax, take your time, enjoy thr drive. It's part of the RV fun.