The limit is for maximum authorized mass (which is basically "what manufacturer says" it can carry max) not current mass.

I doubt anyone will ticket you over being 10 kilos extra...

On top of that you could probably pick any other part to make lighter and come out cheaper than buying fancy solar panels

No you are incorrect. There are 2 maximum masses: the one from the manufacturer and then the legal mass limit. Legal mass limit = MIN(3.5t, max mass according to manufacturer) or more precisely MIN(max_mass_you_are_certified_for, may_mass_according_to_manufacturer).

It is not about + or - 10 kg. But things definitely add up. 10kg here - 10kg there.

Solar panels is something that is under the control of whoever buys the vehicle. You have almost no influence of the vast majority of what makes up the van.

You also risk insurance issues when your car is over weight.

Believe me - weight is definitely an issue.

Who is weighing your van, and when? Genuine question, as this practically doesn’t happen in the US for anything with fewer than 18 wheels.

It happens all over the EU. Police will randomly select semis and camper vans and ask them to follow them to a weighting station. For example in Germany they are spread out along the Autobahn - same in France and NL.

The weight is usually determined by 4 scales (1 for each wheel).

Then they look at your papers which also includes information about max load at the front + back.

If you are over weight you have to get rid of the additional weight and if you can't they will seize the car. You get fined. It does happen.

It happened to me twice in Germany, once in Norway and once in France.

I'm pretty sure that in EU the limit is for the actual current weight, that you can get a huge ticket for an overweight vehicle and that the police use mobile and fixed vehicle scales to enforce that (I specifically remember Austria doing that a lot)

I read an article about it in Czech a while ago, so I don't have a link at hand, but you should be able to find it easily.