I think it depends a lot on what kind of tires you have. They're all different in regards to what pressures work best for them under different conditions. That being said, in my experience, airing down your tires gains you nothing on a gravel road (and could instead make things worse) and won't help you much if it's truly muddy and slick.
If you're not interested in taking along a small compressor to see if any experimental changes in tire pressure are more suited to changing conditions, I'd just keep them at whatever you usually use on the pavement.
Based on your post, I would assume you haven't logged much time riding on gravel? If that's the case, I would seek some out and get some practice beforehand. It just takes some getting used to.
As noted, it depends on what tires you're going to be riding on. If you could let folks know what you've got, maybe somebody with more first-hand experience with those particular tires could provide more useful information.
Sounds like a great trip! Hope it goes good for you guys.