In this case a caravan is a group of vehicles going in together - it's a wild jungle, a little close to the boarder and it's been known to have a bandito or two lurking looking for lone cars and unsuspecting visitors. In a group you should be fine, but it's just too rough a trip on your own.
Personally, I would probably opt for a guided tour for Caracol. The other spots you can see on your own, but this one would be best done in the company of pros - for safety, for comfort, for fun and to get the most out of the journey.
And don't forget to see Cahal Pec! Easiest of them all to see (right in San Ignacio) and one of the more interesting to visit.
The museum is fab.