I went down this road once and gave up expense wise.
Having known folks that tried to get their van upgraded, I went down that path when I had my springs replaced, due to an axle failure years ago.
There is not a simple road in doing this, and a few engineers, advised me of the difficulty. Not in all cases but with many it is imperative to analyses the actual caravans integrity in the first place, and get it in writing that your model can handle the extra weight you intend to carry.
The actual van manufacturer must put it in writing as part of the process and accept responsibility for the proposed upgrade. In other words say that his van shell will be ok with the proposed work.
This path might work with a well built van and to the original owner. Your relationship with that manufacturer will most likely come into play as well. In my experience as the manufacture has to actually accept a bit of risk they are not going to help you.
Your proposed engineering redesign has to be put on a plan and that plan has rightfully to be accepted by the original vans manufacturer. It is a big project for an engineer to just accept any van and add a change to the undercarriage and accept total responsibility.
I mean what is in it for them?
In my case I was better of sticking to my known weights etc. or just go and buy a bigger stronger van.
I now have to note that I cannot accept owners ship, truth, quality, or veracity, of articles I may quote off the internet.
1929 Pop Top
Paul & Trudy. Plumber & Plumberess