What the dealer said was wrong.
Yes you have towball weight, so GVM is decreased. Yes you still have to include that weight in the total weighed GVM.
Weighbridge accuracy is 20kg, because of the increments it can show, i.e 20, 40, 60 etc.
It can't show 50, 55, 56
Even if you ball weight is 180, and you reduce GVM by 210 instead of 300, you are still over.
To suggest it is 170/180 without weighing or seeing it is fraught with danger. The 200 as calculated by the weighbridge is the most accurate you have at the moment.
The best way would be to have a post the same height as the towball (when loaded) put that on the scale and lower the coupling on.
Also looking at you car specs for axle load, their total is only 65kg, (< 1 person) over GVM. so you would have to get your load fairly well balanced.
As GBC said your rear axle weight, is what I'd be looking at if you to do anything. Put front axle on plate, trailer attached and subtract that from your GVM as weighed.
Towball scales aren't great. I've got one and can get reading between 90 and 190 kg for exactly the same setup, depending on how I use it.
I know you said you can't, but anything out of the car to the rear of the camper will help more than its weight. 10 kg out of car is -10 off GVM. Add it to the rear of the trailer, and it will also reduce ballweight, and GVM even further.
In reality you should be fine, I'd keep in mind rear axle Weight for next time.