To add some perspective FWIW, my hobby activity is racing cars and I do participate in amatuer endurance events typically 12 hours long. While you are in the car for up to a 3 hour stint, you may do two of those and are "at it" nd engaged for the full period noon to midnight. Likely with that background:
I have done 18 to 20 hour trips both with the MH and towing an enclosed trailer with the truck. Nothing but coordinated stops taking care of fuel, food, and bio breaks at the same ez-off/ez-on exit, so only off the road for 15 minutes tops every 4-5 hours tops. This usually works out to at least a 60 mph average, assuming traveling with the usual right lane flow of 65-70.
Anything larger than a passenger car does require some amount of increased concentration to drive, but I generally don't find the MH or trailer towing to need a significant increase in my situational awareness compared to when in smaller vehicles. This is probably because I tend to scan 360 degrees continuously while driving in any condition or road anyway.
I think it comes down to whatever one is comfortable with. For some people that is 250-300 miles, or more for ironmen (I guess I'm one) that can do 1250.