Towing with a VW transporter conversion or similar

I’m sure that was a slip of the finger and you realise 85% is only guidance rather than a rule.

I've no direct experience of the vans you mention but, as I read your words, twin cab puck-ups sprang into my mind. Have you considered these? Comfortable car like seating, massive pulling/towing power and huge load space. A Truckman top completes the outfit.

I take it you are going for a "campervan" when you say a conversion,just note that many conversions are on "small" engine versions unless you are going for your own spec ,but  kerb weight wise there should be no trouble towing your twin axle

I took it to mean conversion as in seats and windows as camper vans tend not to have room in the back for the gear the OP wants to stow although he does say with basic facilities......🤷🏻‍♂️

We tow with a Toyota ProAce Verso, its an MPV all the rear 6 seats (combination of 2 x double seats and 2 x single seats) can be removed.

When going away for a long time 2 or 3 months we tend to take out all the seats but we have recently done a 3 week trip and only took out 1 row ( a double and single seat. This still left us with sufficient room to carry, 2 x folding ebikes, a camping twintub washing machine, a porch awning, 2 x plastic type crates, tool box, wheel clamp etc. It has sufficient weight and pulling power so could be a vehicle to look at. There is also a Ford, Peugeot and Citroen version of this vehicle.

If it is a 'camper' type vehicle you are after as you mentioned facilities, we have seen many towing caravans I would guess its down to how heavy your twin axle is.