The official slow moving sign we are all familiar with in the USA (triangle) is to be used on vehicles going "really" slow not just slower than the rest of the herd! I don't know if its legal to display one on any vehicle just because someone is going slower than the rest of the traffic.
Many RVers drive 62-63 in any zone above 60 and 55 in a 55. It saves fuel. Some people are irritated and believe a slower moving rig is a danger...the vehicles going over the speed limit is more of a danger. The real danger is the huge difference between those that go too fast and those that are minding the law. The faster rigs cann't slow down fast enough...yes, a danger but the blame should not land on the slower vehicle.
I think most motorist on our busy highways already realize RVs sometimes go slower and are very quick to pass.