You have already done what the majority folks don't do when they have a question about a CG -- you called the CG. They said no problem, there is a 40' driveway, seems you should take there word for it.
Now will it be a tight spot to back into, most likely. Nearly all CA State Parks have very narrow roads in the CG that don't have a tremendous amount of room to maneuver. So when you get there stop, get out, and take a close look before attempting to place the TT. Make a plan, have a spotter and take it slow -- you will get it.
I looked at some Photo's of the CG and the map, it doesn't look like something I would be overt concerned, especially if I have assurance of the rangers after a phone call.
Backing you TT takes practice to develop confidence. Maybe hitch up and head to a open lot to practice. I did it in a church parking lot on a Saturday afternoon a couple years ago, really helped.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)