You might be overthinking it.
Vent fan for the summer, and a heater with the intake and exhaust isolated from the interior for the winter (dry heat).
I use a solid fuel heater for winter, and in summer a couple small vent fans in the roof to draw the stale/hot air out of the van.
Trying to heat or cool with battery power, aside from fans, is a huge drain on battery resources. You either have to have a very big battery bank with a very big way to charge it (expensive), or be connected to shore power all the time (limits your freedom).