I like 300-400 a day, the truck needs to stop somewhere in the 330 mile range while towing and the wife needs to stop at a about 4X that rate. I have done 650 (BIL's house to mine.) several times, it's not bad but not a preference.
I tend to set the cruise at no more than 70 or right at the limit, whichever is posted.
Edit to add- With the TT, that thing was so aerodynamically filthy that 55 or 70, it got 10.5MPG. It didn't matter. Now once I did pull it home at 75-80 once and got 8MPG. <-- Many moons ago, I don't want to hear it today.
I've noticed (Not done experimenting yet) that my 5er gets about 11 at 65-70, but pulls nearly 12 if I'm on back highways running 55ish. I need to chart it closer and really see.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10