It depends on road condition, weather, and traffic, but generally 70, with or without the toad, on interstates and as close to the limit, or a bit over, on secondary roads. Curves and hills are also a speed "modifier". I've done that since we got our first TC in the later "70's and no problems/accidents related to driving so far.
The last TC we had for 23 years was a Caribou and the MH we've had the last 4 years is a '94 26' Flair, which is older than old according to twinboat.:-)) It has made 3 coast to coast trips so far, and may make another this summer, depending on how confident I am at that time in the '94 30' Dynasty we bought a few months ago.
1994 30' Monaco Dynasty, 5.9 230 HP Cummins, MD 3060, 1992 Geo Tracker.
1996 Dodge Cummins 2500 with 1996 Lance 945 camper