Prevailing speeds change by location and time of day. Best way to determine it is to observe what's going on around you at that time.
In general I don't keep prevailing speeds in my RV. I go less than typical speeds on interstate-generally less than 65. It is easy to pass me on the left.
On two lanes with good visibility I will try to keep up as good pace as possible. Up to 65 MPH depending on conditions and the speed limit. Count on them passing during the frequent opportunities.
On mountain roads the RV is the limiting factor and generally don't go prevailing speed. I simply cannot corner at typical commuter speeds here in CA. People who drive the roads daily (far different than your first time over the road!) and are willing to take a corner at half a g or more force. Best here just to develop a pull out strategy which gets them around you as efficiently as possible.