Hi Cheryl
These panels are fine to prep as you would a normal van. but you must go over it a bit more than normal.
i.e.
Wash and clean down.
Degrease with the likes of a meths, isopropanol, Avery surface cleaner etc etc
bin the paper towel and repeat. you will see you can still remove a fair bit of the dark crap that you get on alloy, even after a good wipe.
once done, give it a heat with your heat gun, not to heat the panels but more to take the cold chill off the surface.
this will help promote adhesion.
once you have the vinyls fitted, heat again to promote further adhesion.
when applying heat, be firm with a felt edge squeegee along the panel joins.
if panel joins are tight, it will be fine, if they are open slightly. then slit between the panels and heat and squeegee firm again.