Hello Robert.
When a caravan manufacture states a MiRO, they do not weigh each caravan so its a best guess based on samples and design information. Very few caravans or cars actually weigh what the manufacture states, they invariably weigh more, due to variations in materials used in the manufacture. Other factors an also make a difference, for example two pieces of wood cut to exactly the same size, will not weigh the same. This is due to the density of the wood, and also the level of humidity. When they fit out a caravan each fitter may leave ust a bit extra on the length of pipe or the carpet, These may only be small variations by themselves but add all the variations and you can get several Kg of variance.
What cannot change, is the trailers plated MAM That is its legal limit and you must not exceede that value, having said that some manufacture do have some capacity to increase the trailers MAM by replating it.
I have nerver actually done it my self, but I have know some people that have weighed every item they put into their caravan and the weighed the whole caravan and found that in reality the caravan was several Kg different to their calculated figure. Take a kg of nails, then weigh each nail seperately and add the values together, the result will almost certainly be different to the nails weighed enmass. One of the biggest errors is due to rounding of values. Errors of 1 to 2% are perfectly common.
I know this doesn't help you but it may explain the root of your problem.
To prove the exworks weight, you have to recreate the exworks conditions. That means removing the battery, and all othe ritems that were not fitted by the factory. Each of the items you have mentioned seem to have whole numbers of Kg for weights, Did the dealer add any items? - radio, liquid in the loo, or water heater, Was the gas bottle full or empty and so on.
What about road dirt, or rain each of these will change the trailers weight.
Whilst the law says you should get what you ordered, its not always as black and white as that. The law also recognises there can be legitimate variations in a product, and I would suggest that the weight of an item constructed from many differnt materials from natural sources may be one of those criteria that cannot be realistically guaranteed.
In my opinion The only way you may have any comeback against your supplier, is if you made it clear to the seller before the contract was signed that you needed "189Kg pay-load and no less" and that was made a critical term of the contract and the seller agreed.
If you did not make it a critical term of the contract, then it is unlikely you would be able to sustain a claim agaisnt the seller as they can probably bring evidence top show the variation is a normal manufacturing tollerance.
In my opinion you have absolutely no recourse against the manufacture as your contract is with your seller.
You may be lucky - in that the Skoda's maximum trailer weight is 1400Kg. That it the mass that is 'towed' technically the nose load is not part of the towed load as it is carried by the car. Thus the gross trailer weight the Skoda can handle is equal to the 1400 towed load + the nose load. I seem to recall that the Skodas maximum permitted nose load is 60Kg thus technically you Gross Trailer weight could be 1460Kg.