You're not really considering the demographic here. Pretty much all of the vandwellers I met are surf or climbing bums and some girls are absolutely into that - not to mention a few were girls anyway. Not everybody is after the clean cut young professional life.
> What would be exciting about going back to a guy's van?
So she can tell her girlfriends that she did it in a Van. Jokes aside, isn't it true that most women just wanna have fun / adventure? So it might be a new thing, depending on the lady in question.