As Bill said, the rust could be from condensation. There could be a leak as well, but being aluminum framed, any rot issues are limited to wood strips for the bag seals which are replaceable if necessary. I replaced one on mine.
As for the rust you see, it could be a leak from any of the openings on the roof. You can remove the trim plates from the inside and check for moisture if it has rained recently. The sealant can look good from the outside, but I found that unless you actually lift the vents and apply sealant at each screw hole and put a bead of sealant underneath the flap of the vent, it may still leak. This isn't hard to do, but it is time consuming.
If the leak is coming from the antenna, it's most likely coming from screw holes. Remove each screw, place a little sealant into each hole and replace the screw.
There is a wood panel in the kitchen area. I did have a leak which got behind the wood panel and rotted it out. I pulled it off and replace it. Again, time consuming, but not hard if you are handy.
Again, as Bill said, the rust you see could just be condensation. But if it is a leak, they are easy to fix and won't cause permanent irreparable damage. You could try removing one of the screws from the window frame nearest that rusted bolt and you may get an idea.
I wouldn't walk away, but if I find that the screws are rusted on the inside, I'd use that as a negotiating tool.
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