It depends, really. Think about, it's 17 years old, got wet and rotted out. Camper is still in one piece presumably and not losing shape. Plus you're fixing the leak.
If it's just window framing, given the above facts or assumptions, I'd be inclined to use good ole cheap spf lumber. Care in connection methods will be equally or more important. I.e. Not splitting or stressing the wood grain with screws. Pre drilling, pilot holes, the proper application of glue, etc.
If it was somewhere likely to get wet again like a back overhang or wing, I'd say use treated lumber for max life span.
If it is critical structure, I'd be thinking hardwood. But idk what's used in different campers. Haven't had the pleasure of tearing one apart. The framing I can see though looks like good ole cheap spf lumber around hatches and openings etc.
Also depends how long you want it to last and the conditions. 80mph into head winds and off road banging around will require more competent repairs than less strenuous use.
I'd say replace it in kind with what's there. And don't use Cedar.
