Hi William,
As usual, both are right!!
The best practice is to remove the wheels and store in a ventilated , sunlight free area, laid down and kept at correct pressure.
However, a lot of Insurance companies insist on wheel clamps being fitted, so removing wheels is not an option.
A better solution is to raise the van on axle stands to take the weight off not only the tyres, but also the main suspension units to allow the rubber to regain its shape and rest.
If you cannot do this, then you seriously have to make an efort and go and move the van every month so that the tyres do not sit on the same area for the whole winter, and avoid deformation of the tyre carcass.
Keeping them covered is to stop sunlight deteriorating the sidewalls.