Depends on your rear gear ratio, but in general, you want to keep you engine rpms in the 2000-2500 rpm range, but I would not want to hold 3000 rpms for any extended amount of time?
For me, that's between 60 and 65 mph.. In WA state, the max speed limit is 70.. If I do that, I'm lucky to get 8 mpg while towing and I'd be near that 3000 rpm mark as well... It's just not worth it to me to go that fast to get to my destination... If I need to get there sooner, I leave earlier..
I have had the big bad diesels pass me going 75+ mph... Funny thing is, they end up passing me again, because they stopped for whatever reason, but not getting there any quicker than I did at that point??
Also, it's what YOUR towing comfort level is going to be... Mine is the speed I go. A lot of others are just fine with the higher speeds, and that's fine too.... As long as you don't end up in the ditch in the end, and you get to your destination safe and not all stressed out, it's a good trip!
Mitch