An evaporative cooler (swamp cooler) won’t be a new idea to some people but it might help others. Living in a smaller space, I can see this working well. If you go to this guy’s YouTube channel, he shows how to make each one in a separate video, so there should be one that’s right for you! (And your pets!)
There’s even a DIY Air Filter that might help those suffering from allergies. It uses a similar method.
EDITED TO ADD: There is SO MUCH useful information on this guy’s channel that’s great for van life! Definitely worth subscribing to or bookmarking as a resource!