Many vandwellers are snowbirds, following mild weather. One major reason for this is heating and air conditioning with electricity from solar power is generally impractical.
Heating is relatively easy with propane or diesel heaters.
Ventilation is also easy with with roof vents, like the Fantastic or Maxxfan. In areas of very low humidity like deserts, swamp coolers can be effective spot coolers. Use of awnings or shadecloth can help keep sun off the van's interior and occupants.