That is a good question. I always marvel when a fellow RV'er passes me when I'm doing between 55 and 65. I know I could run along at 70 to 75, but what is the hurry?
Heading south through Nevada in beautiful weather I averaged 65 mph. That's where my combo is happy.
Coming back north, between two winter storms, I averaged 45 mph. I dealt with heavy winds until I finally dropped out of Jordan Valley and into Marsing.
Finally in Fl 7200 miles for this trip traveling mostly at 67 mph. Truck and trailer happy at this speed fpr any where from 5 hours jumps to 12 hour jumps. Like I have said in the past we do this every year. some times for 5000 miles other times for 7 to 8000 miles. You do what you do. If you do not tow much or often you are not in our league. We do this drives to get point to point not to sight see. Sometimes we go to an area and work it for days. Most times we have been there done that. God knows we love lour country and there is much for all of you to learn and love. Been there. We are retired and do pretty much what we want. Love to all this season.
I do not set a specific speed limit - adjust to time of day, traffic, highway conditions,terrain, many factors, if i feel unsafe at 55 I do not drive it. i have pulled my 35 foot 5th with slide out at 70 and felt- comfortable.However i would not and do not,do so in baja. Your specific situation, determines your speed.I have towed 28 foot TT when 60 - 65 was plenty fast.
Today was Tucson AZ to Stockton Tx. I think with stop for food and a few fuel stops, we took about 11 hours. We ran at 67MPH or 68 for most of the day. When it is mostly flat we cruise when we hit the foothills we slowed for a ways. Going to just east of San Antonio tomorrow estimate 9 to 10 hour burn. Mostly flat some hills 67 mph, likely. 5500 miles so far on this trip. 20 days , mostly to visit family this time.