from my experience ......as for tow speed i would say what ever speed and conditions you are comfortable with, reason i say this, it is nicer to tow on a roads and dual carridge ways than the motorway at times cos the motorway has those annoying ruts that can swing the car and boat all over the place.
have towed the large caravan, largeish boat and the rib, the rib is the most stable and just sits there at any speed, i wont say what i tow at but it always feels very very stable. for example i hate towing the hard boat on double axle trailer faster than 50 but the rib just sits there solid as a rock on a single trailer at any speed. the caravan can wag around a bit as richard quite kindly knows!!
when you come to hills it is sometimes better to drop you speed a bit while going down to keep control, i often go up hills faster than i go down em.
make sure your tyres and bearings and pressures are good, jockey wheel is tight as they can rattle down, boat well strapped as they can flip or bump round and your nose weight is suitable for the car, for the beemer i keep the nose weight very light, just heavy enough so it does not rattle