I do a lot of work on ex ambulances being converted, both the box bodied (modular) dually types and the PVC types.
It's truly staggering how many people buy them naively thinking they can down plate a dually down to 3500KG and actually have any form of payload at all!
My own modular Sprinter Dually once stripped down was around 3300KG empty and weighed in at 3960KG built with a full tank of fuel and the driver, fully loaded on trips its normally nearer 4400KG.
The PVC types often have air suspension as part of an uplating and uprating exercise e.g the 4000KG Ducatos
I also have an uplated Crafter with VB Air assist and a stronger front spring which took it up to a max of 3850Kg.
I mentioned the latter because it seems very common in the van community to add air assist rear springs when the vans hitting the bump stop a bit too often but I wonder how many people do the right thing and get it weighed first as part of that exercise!
Maybe the damn things was simply to heavy in the first place etc.
I suspect many many vans out there are well over 3500KG when on the road especially if carrying the baggage of multiple passengers despite all the consequences that could entail if stopped or worse if in an accident
I can't echo enough the previous comments, to get any potential purchase weighed, ideally with a full tank of water however if only using campsites the water tanks are probably the one thing you could leave empty if it makes enough difference.
Don't forget a lot of vans don't have spare wheels some don't even have jacks so check!
I wouldn't be without either nor a small toolkit and I also carry a couple of waffle boards which I have needed and used!
Most off the shelf vans don't have
solar panels or they are so inadequately sized as to be pretty naf, so if planning to off grid allow for weight they add etc. Some people take Bikes or Boats etc as they are fundamental parts of their van trips.
I.e always factor in what you may want to add to the vehicle and/or what you consider to be a minimal basic emergency kit.
Lastly whatever you buy if you find you are really tight on the weight limit always always take everything out of the van after each trip and decide if you actually need it next time it's amazing how much weight accumulates in a vehicle if ignored, especially electrical gadgets, books, excess cameras, outdoor furniture, food cans and cooking utensils etc.