Towing a caravan with a diesel Multi I can easily get to 60 mph on level roads, but I feel the outfit's pretty close to it's safe limit at that. It doesn't take much of an incline to drop the speed rapidly though. Lots of early downchanges helps.
Towing a bike trailer however, 70 feels quite comfortable, you'd hardly know it was there. Must be the greater wind resistance of the caravan makes the difference.