It's a little hard to tell from the photo, but what you've got there basically is this, moving from outside inward:
--Outside fiberglass shell
--Some expanding foam stuck to it
--Air space of some amount because shell is shaped and wallboard is flat
--1/8" vinyl wallpapered hardboard type wallboard (vinyl is built onto it not put on by Chinook)
--A strip of corian backsplash (separate from counter top) held on by silicone (yup
)
--A 1/2" corian countertop
I'm guessing you're asking about the rusty/brown areas at the top of the backsplash? If I had to guess I'd say it's either dirt/iron from water or something else built up on the silicone, OR the window could use re-bedding (not an unusual thing after 10-15 years).
Could there be worse horrors? Possibly, but it's more likely to be one of the things above, IMO (from one photo only of course).
Worse horrors:
1) Window has been leaking tons of water for years, water ran down inside wall, floor is rotted. This is highly unlikely in my opinion. First of all, it would smell pretty musty I'd think. Second, even if it's of the era of the plywood floor, the floor in that area has quite a bit of fiberglass over it (because the wheelwell is bonded in place with fiberglass cloth, then so is the fuel filler). And if it's the foam sandwich floor, sure water *could* get in between the layers, but not likely without some further signs.
2) Roof has leaked, water has run along some channel or gap, run down the wall, discolored the backspash, then gone on down to cause horror #1. Again, this is extreme and probably unlikely.
As a first step, I'd check these things out:
a) Does it clean off? I see some of the same on the side of the counter by the little mini-divider that separates the couch area. That latter area is extremely unlikely to be leaked on, so maybe it's just gross silicone?
b) If you look under the sink, does it smell musty? Can you see any evidence of water leakage back by the wall? Is the wooden framing that supports the Corian top in good shape? Carpet stains?
c) On the outside, has anyone globbed sealant on the outside of the window trim ring? (This never seems to actually work, but people try it anyway.)
Chinooks have a huge and everlasting leg up on most other RV construction because there are no seams in the shell to flex/move/leak. OTOH, there are still penetrations that need to be re-bedded from time to time. Roof vents and rails, windows, hatches, etc.
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