I own a 2013 Sightseer 35G which came with a fireplace. Unfortunately it went out, shorted out. I called Winnebago looking for a replacement and they said they no longer carry replacement for my fireplace, they gave me the number of the distributor who also said they are no longer made. So I did a little research and settled on a Puraflame 26" fireplace. I bought it from Amazon and had to build my own mounting brackets as they did not come with any. Anyways, here's what it looks like in my rig.
Hope this works!