Driving my 31 foot Class C with our Chevy Aveo on a tow dolly from Little Bennett County park near Fredericksburg, MD to my uncles 90th BD party in Leesburg, VA, I was blindly following the GPS when it routed me to White's Ferry over the Potomac.  I thought, ok, no problem, then when I got around the bend to the ferry with no place to turn around, there was a 60 degree down angle to the ferry, then 30 degrees up on to the ferry.  The dolly popped right off the hitch and the car went 2 feet to the left blocking 1/2 that lane for the ferry.  It took me and 3 guys 10 minutes to maneuver it back on, then it promtly popped off again when the had the same set up getting off the ferry and driving up the other bank.  A word of caution to other newbies who have dollies, learn from our mistake