This imo reinforces my position that unless there is another towing weight limit then the plated weight is the law.
In the event of being weighed the motorhome would be weighed in total and by axle and the trailer would be weighed . The vehicles towing limit being the difference between gvw and gtw could not be ignored.
1. The capability of the towing vehicle
The chassis plate on the vehicle (see table in Section 7) states the maximum weights allowed - the Gross Vehicle Weight (GVW) and the Gross Combination Weight (GCW).
The vehicle handbook will either repeat what is on the chassis plate, or for convenience, might directly specify the maximum weight of trailer (eg 750kg) which is allowed to be towed. The V5C registration certificate often shows this too, under sections O1 and O2 (depending on whether trailer has brakes or not). Exceeding any of the above weights is likely to be construed as using a vehicle in a dangerous condition.
You keep stating this as though it is fact; it is not and is no more than an assumption that you keep making.
I might just as well say that the Earth has two moons; if there is one moon it is reasonable to assume there must be a second. It is not so.
It is true that the towed vehicle must also not exceed its plated weight but this is just another plated weight that must not be exceeded and it is unrelated to the GVW and GTW (GCW).
Ian