It is in fact for Sirius/xm radio. You may want to look around in the rv for the box that goes in there. To install, push in lightly on the top and it will rotate out slightly at the top just like a cassette player. The box than slides down into the holder and push slightly to lock in place. I did take pictures of the one in my 2011 Flagstaff 8528CKSS which I believe is the same as yours. I read this from my phone but can not find the topic on my home computer. If I find it, I will post pictures of it installed and uninstalled.
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Flagstaff 8528 ckss fiver
2008 Ford f350 dually 4x4 desiel