I think Montana has it. Other states are 75. In our car going 80 is no problem. In the RV we prefer 55 - 65 and just stay in the right lane with the trucks.
Funny thing I noticed driving an RV: 5 mph made a HUGE difference in fuel used. 60 mph got ~7-8 mpg. 55 mph got 10-12 mpg. I started driving at a considerably reduced rate after that discovery. I just started 20 minutes earlier, and drove 20 minutes longer and got there more relaxed.