You're right! Cutting off too much could seriously impair the functionality of your unit. After you get the matter of access taken care of, put your hand in and feel around for the approximate center. Then take a 3/8 bit and drill a hole. Use your SAW to enlarge the hole bit by bit until you can reach in from the outside and feel the inside perimeter of the mounting plate. After that just keep on going till you get the results you are looking for. I'll try to check in with you tonight to see what you think after you have time to check it out further.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.