mwilson looks like I don't really need to upload my RH Truss family. I have tried with a standard Howe Flat Truss in the Revit 2016 template and the same thing happens. If I use the code on "W Shapes" it works as expected. Is it a bug where Revit can't access these parameters in a Truss family?
Edit: I can confirm this also happens in Revit 2017