I would recommend to use Kelvion air to air heat exchanger. It is available in different designs and sizes. You can try out this product for better performance.
So why run outside air through an exchanger instead of just blowing or sucking it directly into the vehicle? If you're going to raise the temperature of incoming air, what with? The air that's already in the vehicle?
With the air that's unavoidably forced out of the vehicle to make room for the incoming air.
What's to keep the cool outside air from cooling the warmer inside air? Or if you're trying to cool the interior, what's to keep the warm inside air from warming the cooler outside air?
The heat in the outgoing air is transferred to the incoming air by a device called a counter-flow heat exchanger.
You're absolutely right that running continuously would heat/cool your space to whatever the outside temperature is, but you get a lot more ventilation before that happens.
I use a Russian-made* heat recovery vent instead of a HEPA filter. Right now the temperature outside is 8 degrees and the inside temperature is a stable 65 even though I'm exchanging 20 cubic feet per minute with the outside. It runs in a small room that's sealed from the rest of the house and doesn't have its own thermostat, so it's not getting any extra help from the furnace.
(It's set up with four 60mm computer case fans split between the intake and exhaust to keep the flow balanced, but in a van you could get away with just two on either the intake or the exhaust. There's enough room for three fans on each port. Power draw is 2 watts per fan.)
