If you take a look at the photo (source: still from this YouTube video), you'll notice a pair of "panels" on the fuselage of this Omega Tanker DC-10 (reg: N974VV), circled with green.
There is also a pair of "portholes" (circled red).
What are all those? Any relation to this being a tanker?
Note: Here's a photo that shows the "panels" closed on the ground.