Sorry I wasn't sure what you (original poster) was meant by toggle-menubar, so firstly though using Alt+E I get the Edit menu opened so you can use the keyboard to pick an item. Likewise using Alt+F for File, Alt+V for View etc... I tested this on 19.04's
pcmanfm-qt as it was what I'm running at the time
@DKBose however felt you were after the means to toggle the menu being visible (using Firefox as an example. A quick look in the shows View->Toolbar->MenuVar shows the 'menu' can be enabled/disabled (or visible/invisible) using a checkbox or ^M (Ctrl+M) on my
pcmanfm-qt (DKBase confirmed it was the same in 18.10; I can check 18.04 later today using
pcmanfm but I'd expect it to be).
A quick play whilst invisible showed that Alt+E didn't pull up the Edit menu as I expected (likewise other Alt+menu-entries) until the toolbar was turned back on (or enabled) which may not be an ideal solution, but it's mostly there. (my testing was limited; it was right before I went to bed...) It was not difficult though to just ^M first then use ^V to use the view menu option.