Judi, we removed, cleaned up and re-installed the rear window (Hehr series 4900) on our Trillium
4500 last year.
vintagetrailergaskets dot com shipped us samples of the series 4900 gaskets free of charge. We decided to order and replace the top seal and all of the Y seals, but not the glazing channel gaskets. The glazing channel gaskets seal around each window pane, between the raw glass edge and the aluminum.
We used a thick layer of butyl tape behind the window flange and have not had a problem since. However, also consider (as mentioned in a recent post) that a gray sealant is used to seal the joints in the aluminum frame. After 40 years, this sealant may need to be replaced.
To answer your question, YES the glazing channel gaskets can be replaced without removing the window. To do this, open the window wide then remove each window sash by removing the two screws on each side that hold the window sash to the "Hehr torque bracket". Then disassemble the window sash, extract the old glazing channel, etc.