Boy oh boy is this a complicated topic. There is no clear solid consensus on what the “best” way to insulate a van is, but there sure are a lot of opinions with very little in the way of empirical data. At best, you're making an educated guess and almost everyone insulates their van differently.

FarOutRide's guide is a good place to start.

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. But it will help when aided with either heating or ventilation to keep the interior more livable.

It is better to insulate the entire van poorly than to only insulate part of the van well.

Windows are going to be the largest source of both heat loss or heat gain. Especially If you have a van with windows all around, it's pointless to do any insulation to the body at all if you're not doing something about the windows in the form of insulated or reflective window coverings.

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.