My MH is right at its GVW of 20,500 lbs and we're pulling a 5000 lb toad. We're done that for probably 40,000 miles without problems. We are currently headed back to Minnesota on I80 from California. Some rather long and steep grades coming over the Sierra and in Wyoming. There have been several times when we've slowed to 45 mph (V10 at 4000+ rpm in 2nd gear) and even had to drop into first a couple of times at 30 mph (4500 rpm). Those were at higher elevations where a NA engine takes a hit. Never any doubt, however, about making it to the top.
Today we were on cruise control at 65 mph across Nebraska (could have easily pulled 70 mph, but the mpg takes a big hit).
And that is all with 310 hp and only 4 tranny gears.