Sorry to jump in on this topic, but in reply to the OPs question regarding single vs twin axle ‘vans, there are a few other things you might want to consider.
While a twin axle will be inherently more stable, a correctly loaded single axle should not be unstable and with a properly maintained stabiliser hitch you should have no stability issues with a correctly loaded van and sensible driving.
A twin axle van will likely also be significantly heavier, towing with the XC90 won’t give you any issues, in all likeliness, but if you don’t need a heavy van it’s a lot easier to make good progress with a lighter van - and you’ll get better fuel economy too
It’s also worth considering that a twin axle van will be more difficult to level correctly on site. I’ve just come off a site that was very hilly, it was hard enough to get the van laterally levelled for a single axle, I would have hated to have to level a twin axle on the same site.
That also touches on the subject of manoeuvring the van. Do you intend on having a motor mover fitted or do you hope to be able toise manpower? Single axles are far easier to manoeuvre and can rotate on their axis - something which is much harder on a twin axle van, although I have seen it done with enough brute force and ignorance.
Servicing costs will be higher on a twin axle van, too. Twice the brakes, tyres and associated bits and pieces. If you’re having a mover fitted then obviously a device that moves all four wheels will be significantly more expensive than a mover for a single axle van.
It probably seems like I hate twin axle vans but if I could have afforded an Avondale 630 when I replaced my van I would have had one without a second thought. Nicely built van with a great layout and all in at 1450kg fully loaded!
I agree they can be more spacious and offer some layouts that can’t be achieved with a single axle, though.
Some continental vans offer far better layouts than British vans, and they are better built to boot. That’s the route I went and I found a German van with fixed twin bunks at the rear and a fixed double at the front. Checkout Fendt, Knaus, TEC, LMC and other vans, they do make some massive T/As but they do some sensible smaller vans with great layouts too.
Hope this helps some.