Nobody in the US will know that screw as a "Robertson" head. the term used here is an internal square drive head. the tools are common. Less so the screws.
I find the easiest way to get these is to go to a smaller RV repair shop and buy some off of the guy. I have been to numerous Fastenal, Tacoma Screw and others and have yet to find them. But they can be ordered from RV parts suppliers by the hundreds.
I often use in replacement a star drive screw if it is not on the outside of the body.
As a last resort you might try to order them from McMaster-Carr.
