You have to turn the locking ping with the slanted side out or it won't go all the way in. Also, I have to install mine upside down (from the bottom) to have clearance on the base plate. Just my findings, others may vary.
Dan & Ro with two Rescued Boston "Terrorists"
2015 Tiffin Allegro RED 38QRA
Previously '06 Itasca Sunrise 35A on a W22 Workhorse
'12 Jeep Wrangler Sport and/or '08 Honda Goldwing