Good question. At the Heartland Rally in Goshen this summer, the Lippert people said to jack on the axles. The Dexter people said to jack on the frames. The general consensus was that you call Superman, who will pick up the entire rig and hold it in the air while you change the tire.
Seriously, how hard would it be to provide that sort of information? I would assume that on the frame, as close as possible to the axle attach point would be a good place, but I don't know. Have you asked your dealer or Heartland?