Seems to me that this is highly traffic and terrain dependent, with a good deal to do with your towing speed and your rig's capabilities. I tow a smaller trailer (23') with a diesel, so my torque to weight ratio is good and I can accelerate with many cars - thus my experience may not match yours ...
I tend to drive at the local speed limit but unless there's a lot of slower traffic, I stay in the right lane as much as possible. This, however, gives me two disadvantageous results: first, in areas of heavy traffic (i.e. Atlanta, Chicago, L.A.), I sometimes get "stuck" in the right lane and can't pull out to pass for miles; annoying, but I just put up with it. And second, that means I do have to pull out to pass slower trucks and slower RVs a lot, especially on steeper mountain upgrades ... but it also means that while I'm in the right lane, I'm out of the way for those leadfooted left lane speed demons. It also means that on longer downgrades, when I'm letting the tow/haul function control my speed, semis going at seemingly crazy speeds sometimes/often pull out to zoom by me.
I think ya' can't have a fixed rule, but literally have to go with the flow.