Hello Ann,
Motor manufacturers test and establish what the maximum allowable mass (MAM) of the vehicle can be. That in turn defines what he maximum mass of trailer can be. It is not a secret and it is quite easy to find out what most vehicles are technically able to tow.
There is a lot misinformed comment about percentage mass that can be towed, and two figures keep coming up, 85% and 100%.
The 85% is an arbitrary figure and is purely a suggested guideline put about by parts of the caravan industry. It is in my view a sensible target, but it is not enshrined in any law.
100% is part of the current driving licence restrictions, it too is an arbitrary figure but and has no other bearing on the technical capability of any vehicle. It is used because it is easy for the authorities to check against the vehicle data plates on roadside checks.
You must consider that virtually all articulated lorries tow at well over 100% sometimes as much as 500%, but that is well within their capabilities because of the mechanical arrangements of the tow hitch (5th wheel) and the way the weight is transferred from the load to the tug.
Provided the vehicle is rated to tow the desired load, and both the vehicle and trailer are in full working order, then the limiting factor is the driver - who must be sensible about loading, choosing the best route, and as ever choosing to travel at a speed that will enable full control to be maintained.
If I had a large load to move, and I knew of a friend who had a suitable vehicle then I would be happy to ask, but I would also accept a refusal if they were not confident about the task.