The dynamic coupling between the trailer and tow Vehicle and the relative roll centers, weights, spring rated, types of suspensions etc all make a difference.
The loading of the trailer and then % of the CG between the trailer axle
and hitch makes a big difference as well.
The longer the distance from trailer axle to hitch and the shorter distance from the hitch to the tow vehicle rear axle the better.
The dynamics are probably too complex to cover in dozens of posts.
If you start to detect problems at 65 mph I think that you are not setup just right.
Keeping the weight
concentrated close to the axle of the trailer is good and conventional wisdom wisdom in the US is 10 - 15% of the trailer weight on the tongue is hard to argue with, but some here think a well setup rig can safety operate at 7%.
I don't recommend it for anyone else, but this is what I shoot for to keep the loading within the limit for my VW.
My trailer has had the tongue extended 18" for a number of reasons and the VW is setup with T-ESP, brakes
As a caution I have not towed this new setup yet as I am just finishing a major rebuild and upgrade (I hope) and we will be on the road the the Green Eggs and Ham camp in later in March. We will have some experience with the enhanced setup by then.
I aslo have a scale for the tongue to make certain that the weight is what I think it should be.
VW recommends having the tongue weight at it's maximum limit and I set the rear tires
at a higher pressure than normal driving.
It is also important to note that the hitch weight must take into consideration the loading in the vehicle as well and overloading the TV and adding the trailer may very well surpass the rear axle limit so pay attention.