Insulation can help retain heat in cold weather, and reduce transfer from hot body panels in direct sunlight in warmer weather.
Insulation will not, by itself, make your van cooler than ambient in the summer or warmer than ambient in the winter.
Non-insulating folks in mild climates may want to use some insulation on the interior of the roof underneath the solar panels. Heat re-radiated from the underside of the panels may increase roof temperatures even though the roof is shaded by the panels.1)
See this write-up of testing floor vs ceiling insulation.