I've seen that on quite a few Chinooks. I'll explain why I think it happened there.
See the strip of plywood just to the left of the opening in the first photo? See how it ends in a certain place? And how that corresponds with the crack?
Well, there are two main "bulkheads" (sort of like bearing walls in a house, but in a fiberglass boat sort of way) in the Chinook. The one between the sink and the shower, and the one between the stove and the refrigerator. That little strip of plywood is fiberglassed to the hull (wall), and then the bulkhead is attached to that. So there is some pressure there, especially when driving. Then there's a cutout (fridge vent) right next to it. Now bounce down a few dozen roads, and .... crack-a-roo. It makes sort of a stress riser, IMO.
When I saw how that was put together (I had the fridge, stove, and all those walls out for a remodel), I added some structure below that fiberglass strip (at least that's the simple description). I also reinforced that "square" around the opening.
Now, that crack may only be in the gelcoat, because the gelcoat is brittler and less reinforced than the actual fiberglass skin. So it often cracks first. And gelcoat can be repaired. But then too, it can crack again if the same reason is still there. And the fiberglass itself may be cracked (although it's likely fine).
It's sort of a pisser that it happens to be in one of the areas that also has painted stripes

Natch
Anyway, if you want to DIY a fix, we can discuss further. Or even if you want a pro to do it, I always like to know the basics before I talk to them, if possible. Maybe you do too. There is a range from "do nothing" to "stop it from moving any further," to "strengthen but leave the cosmetics alone," to "strengthen and fix the cosmetics."