Increase wheel size on camper

What are you hoping to achieve by fittung larger wheels?

Steve it will depend on the overall diameter & width of the new tyres. I'd check how much bigger they will be then measure from the tyre to the mud flaps, the gaurd & lip of the wheel arch to ensure clearance. If the tyre is conciderably larger then it's possible it could touch on the inner gaurd as well when the suspension bottomed out before the bump stop. In this case the suspension would need to be raised by the increase in the tyre radius. Larger tyres will also reduce the effect of the brakes although in this case I'd suggest it would barely be noticable.
Tyre size is also stamped on the compliance plate so technically would need to be reregistered plus the higher stance would make the van slightly less stable hence the manufacturers hard line.
Cheers Craig...........

Not sure if you have considered fitting larger tyres on the trailer, so that the final diameter is the same as on the 4x4.

Might be a way around the "wrong wheel" warranty issue, and then you can use a vehicle spare if needed, and you might also be able to use the trailer spare on the car, at least on the rear if the 15's won't fit over the front brakes.

I bought a new Goldstream Wing II last November, and it came with 16 inch wheels. I had specified that the wheels were to match my towing vehicle (A VRX Pajero). I rang the manufacturer (about a different issue) and mentioned the wheels and he seemed quite happy.

As it turns out, they didn't match, because the Pajero has 17 inch wheels, but that's a different story. ( I had no idea at the time, as I picked up the new vehicle and the camper within two days of each other, and relied on the campervan dealer to know their stuff.....they didn't. They certainly were not interested in the problem once they had gotten their money. Great campervan, though.)

Cheers
Maaate