If the road topography is flat enough where the truck won't need to be shifting much, set the cruise at low to middle 60s, around 1800RPM because that is where it stays in top gear comfortably, & whoever comes up behind has to deal with it. On a two lane a trucker may get some help in the form of a slow down when a passing opportunity comes up but cars have plenty surplus hp to get around without help.
I actually do have a custom bumper sticker on the trailer back which says "No, I am not in a hurry". It must annoy some of the tailgaters.