We had a reservation for site 59 but did not stay. recreation.gov says that this site can accept trailers with a max of 37 ft and while that may be technically true, I think that could only be accomplished if you could somehow have your trailer carried in and dropped into the site by a helicopter. The loop road around the campsites is simply too tight to back a trailer into this or pretty much any of the other sites along these loops. The actual RV sites are 82-112 so stick to those. 

For reference, we have a 30' trailer and tried going at it from both directions, but the road and the bounding trees are just too tight to make the turn without scratching up your truck or trailer. The people in spot 60(right next door) had a tiny R-Pod trailer and said that the only way they could get their trailer into their spot was to make a circle by driving through 59 and then around into 60(technically driving on vegetation that would probably break some NPS rules). We ended up canceling our reservation and driving way back down the road to Thousand Trails San Benito. 

Thanks a lot, recreation.gov for the inaccurate information, the frustration, and the wasted time. 

TL;DR If you have an RV, do not book into any sites here other than the designated RV sites, which are 82-112.