Thunar doesn't actually have a keybinding for changing to the next/previous tab, but instead the underlying GTK control, in this case GtkNotebook, has some key bindings.
GtkNotebook defines previous tab as both Ctrl+PageUp and Ctrl+Alt+PageUp (and similarly for next tab/page down), however in Thunar's case the Ctrl+PageUp/Ctrl+PageDown keybinding is blocked by the fact that the pane you're viewing (the icon and details view) is of type GtkScrolledWindow and has its own usage of Ctrl+PageUp/Ctrl+PageDown decribed here:
The last time I installed an OS with Thunar on it, it didn't even have tabs so I immediatelly discarded it and used PCManFM, which although it doesn't define a next/previous tab key, doesn't use a GtkScrolledWindow for the viewing of the files and therefore the built-in Ctrl+PageUp/Ctrl+PageDown for the tab widget works.
IMO both programs (and all file managers) should define these, and use reasonable defaults, in addition to supporting move tab left/right like many browsers do (Ctrl+Shift+PageUp/Ctrl+Shift+PageDown).
For Thunar, I have a patch I made to at least define all 4 of these events as keybinds explicitly (which would make them overrideable in the way you described in your original post) that I hope to post soon in this bug report:
Unfortunately while it does let you change the keybinds for those events, it doesn't let you override existing keybinds from child widgets (such as GtkScrolledWindow), so you'd have to use and bind something like Alt+PageUp/Alt+PageDown or equally messy. If I figure out a patch for that, I'll definitely post it to the bug, but that's proving harder to figure out.
* *Edit **
You can move to a specific tab using Alt+N, where N is a number 0-9, but I find this to be of very limited use.