After more hours than I care to admit I finally found out how to type ç on an US Keyboard when using XFCE4 (may be applicable to others).

"Alt Gr + ," followed by "c" will produce: ç

This actually makes sense as stated by yanglifu9‘s in a greatly underrated post here
“It is natural that 'c will produce ć since on the layout the key ' is shown as acute accent ( ´ ).”

See also: http://dry.sailingissues.com/us-international-keyboard-layout.html