If you have an accessible vertical interior wall in the garage (not exterior) make or get made up a steel plate of approx 150 mm square and 3 to 4 mm thick, with a 4 mm hole at each corner and in the centre have a 15 mm hole with a 12 mm nut welded over it.
Select your most convenient fixing place for the wheel by standing it on floor in your chosen spot and mark through the centre hole of wheel onto the wall.
Uusing Sikaflex or stixall or similar on the fixing face of the plate secure with 4 screws onto panel.
Once the plate is secure in place, stand the wheel over it and measure from the plate to the outside of the centre hole on the wheel, obtain or make a plate big enough to cover centre hole of wheel and drill a 13 mm hole in its centre, using the measurement from plate to wheel outer add the length of nut welded to plate and the thickness of loose plate to go over wheel centre hole, cut a piece of 12 mm studding to length allowing for the extra nut that you will use to secure it in place.
The reason for using Sikaflex etc. as well as screws is that the walls of van will be a flimsy composite panel that screws will not get sufficient purchase on, the glue will give a large (2250 sq mm) surface area of attachment.