The tool “ARandR” lets you position your screens nicely. But you cannot assign a primary monitor – the one where the panels and menus should be on. This can be resolved by adding setting the “output-name” property of the panels. Either do it in the “Settings Editor” or from CLI with:
xfconf-query -c xfce4-panel -p /panels/panel-1/output-name -s LVDS1 --create -t string xfconf-query -c xfce4-panel -p /panels/panel-2/output-name -s LVDS1 --create -t string
Attention: The names of your panels might be different. Some examples on the web use panel-0 and panel-1 instead of panel-1 and panel-2 like on my Pentoo system. Use “Settings Editor” to check what names are used!