Should you be concerned? Certainly. Should you automatically write off a ca. 20 year old RV because of evidence of some leaks at some point in time? In my opinion no, but it does warrant careful inspection and thoughtful consideration.
I would suggest pulling any skylight/vent trim rings (on the inside) in the area of the staining down to inspect the internals of the roof. This is very easy to do (a few screws, literally a couple of minutes of work) and should give you a better idea of the extent of damage and the structural soundness of what is there. If there aren't skylights but there are other things like lights or A/C vents mounted to the ceiling, it may be possible to quick peek inside easily by removing them, too—but it also might not be possible to tell much of anything.