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.