Are you saying that your awning does not already have support arms?? They are included (free!) with every trailer -- otherwise, there is no way to deploy the awning.
Maybe someone removed the support arms from your trailer?
The 139 series trailers have a sliding awning "rafter," which I think is the same thing as a support arm.
Finally, another question -- when you mention the drip "rail," do you mean the track where the awning meets the side of the trailer?