The TabStop property set via the property builder of VS should be able to set the determine the tabbing behavior of RadTextBox and more specifically, the behavior of its inner TextBoxItem. Right now, the developer has to use a line of code to determine this behavior:
this.radTextBox1.TextBoxElement.TextBoxItem.TabStop = false;
while the TabStop of RadTextBox is always false, so that the focused can be always transferred to the inner TextBoxItem.