TLDR: They are behind the seat, and are accessed through the radio hatch on the left hand side. some camera clipping may be required and can be done through the window.
What Is this Mission?
Change the air tanks they said. It will be easy they said. I tore the entire plane apart looking for these blasted things.
The only thing that needs to be done here is to change the tanks. that's it. You can ignore the bit about cleaning the machine guns.
Where Is It?
Pull the radio out.
Get the camera in here. (It's a pain).
Behold the tanks! (Only the red ones can be changed). Now just get replacements and do it all backwards. No gun cleaning required.