We were specifically asked for the permission document because the registration was in the name of the LLC. It was just a bit of paper drawn up by myself as LLC member giving myself permission to take it across the border. Included a picture of the MH to make it look a bit fancier. Wasn't notarised, but without it we wouldn't have been cleared.
I've since changed the name of the LLC to our full names so the registration and title has our names on it but of course with "LLC" at the end - hoping to make it less of a hassle in Central and South America.