Tension or compression would depend on the stiffness of the walls.
For example: If the walls were made of concrete or stone like in an old church then the truss would only be in compression. Likewise if the walls were flimsy and relying on the roof to hold them together then the truss would be in tension. The arches would almost have to be made from two pieces which means there will be 4 joints being pulled apart. If there were keys and bolts fastening these joints then I could see this working as a tension truss. This is just what my intuition tells me.