The window trim is no issue, almost all of them do that with age and it is cheaply replaced (though a bit fiddly to do). It does not affect the watertightness of the window.
The sagging roof - depends. While these are fiberglass shelled, there is wood used between the fiberglass and interior paneling in many places, including around the skylights. The fiberglass and foam will not deteriorate with moisture, but the wood will. If the skylight has been leaking, it is possible the water has run elsewhere, it can run a long way from the source of the leak in these campers. If the current owner let me, I'd pull the trim off of the interior of the skylight, you should be able to access the wood surrounding it in the gap between the skylight spigot and the paneling. Poke at it with a bent nail, probe, or whatever you can get in there. It may be obviously wet or soft, or may be soft when poked, indicating moisture and rot.
The roof on a Bigfoot can sag or distort or delaminate a bit without any help from moisture intrusion, if not excessive I'd call it par for the course at that age. If there is significant rot to the reinforcing wood in the roof, it is repairable, but expensive to do, and hard to find an RV repair shop with the skill to do it.
