Interstate speeds

I will set the cruise around 65MPH, no issues with speeding up a bit to pass if needed. Tires are Sailun S637's.

Listen to your truck. "My 3500 Dodge seems to like 65-70 when pulling this 39ft TH." You will probably be more relaxed as well.
Every vehicle has that sweet spot, more stable, uses less fuel.

Ken

Usually around 65, maybe up to 70 if we're out on I-80 in Wyoming and it isn't windy. But really no reason to go much faster for the reasons everyone else has already stated.

I tow the 5er at the posted limit up to a max of 70.

Cruise at 65 and pass up to 70.

I am just not in that type of rush to get anywhere. Back when I had my F350/Raptor combo I had to lock it up to avoid hitting somebody. I just can not find any reason to be going 70+ in my RV, plus it kills my MPG.

Not a 5er here, but one bodacious truck owner here with a longggggg travel trailer.... take your cue from the other 5er's "out there". 60-65 seems to be the sweet spot for all towing.

Always remember, it's not how much or how fast you can tow... it's how SAFE you can tow and maintain 100% control of your rig, under any condition.

I suppose that safety would be first consideration. Are your tires ST type, if so they may have a speed limit of 65mph. Quality of components and age are another safety concern.
I have ST tires, and have excellent luck with them. My previous tires were rated to 65mph, my new ones are rated at 75mph. That is nice information; but I have no good feeling about dragging it down the hiway at 75, even though I have upgraded springs and related components. And my brakes are in great shape, still, it always comes down to being able to stop and or react to what gets in your way. imho

For the 22 years I have been towing a 15,000lb 38ft 5th wheel I've had luck at 60 to 62MPH (in 55MPH towing zones) never been challenged by a state trooper/ highway patrol. In 70MPH towing zones I still stay at or below 65MPH for safety more than any other reason. My c4500 is smooth at 75MPH towing but I feel safer at the slower speed after seeing some results that occurred to other 35 to 40 ft 5th wheels at speed.

The speeds you are talking about are a little scary for a couple of reasons:
> Primarily tire heat for the TH. Blowouts aren't pretty.
> And being able to stop in time and/or avoid idiots

My first question is what are your TH tires rated for. My 42’ 5th Wheel has tires are rated for 75 mph I cruise at 65 and go to 70 to pass if necessary. I would not run at the rated speed for a long period of time.