I believe that is the same model I have (see pic from internet). My trailer is in storage so I can't tell for sure, but it seems to me I just pinched/rotated that knob at the bottom to remove the cover. Wish I could be of more help here.
__________________
Karen Hulford
2013 Escape 15A, "Egbert"
'93 Ford 150 XLT or
'22 GMC Acadia Denali
The little expander knob at the bottom was sticky and I didn't want to force it... made of light plastic & could break easy. Jiggled it back & forth as Karen & others suggested and it came loose.