Cool camper.
This is a
Shasta water fill pot. Your one, at bottom right, is another version.
EDIT: BT is correct: The thing I thought was a furnace pipe is a pressurized water connection. (Found a photo on line.)
Not sure what the cap with the chain is. If your camper doesn't have grey tanks, that might be a place to attach a hose to drain your sink outside? Might be easier to tell if you posted a photo of it open.
(Thanks for the Courtesy Clickies, gbopp)
G-half can always find a way to do things upside-down, inside-out or backward.
It's his Super Power!