Most people go out and buy everything new. The Boss of the Swamp (popular YouTube channel) shows how he fixed up a gas refrigerator and water tank that he salvaged from an old Shasta camper.
He goes into more details in another video: How to Outfit a Cabin on the Cheap From the same $400 camper he will use the sink, furnace, breaker box, outlets, door, windows, vents, plumbing and other salvaged parts. What a bargain. The gutted camper will be used for storage, And if he ever wanted he could strip it down and use the trailer. I love listening to old timers like this who have been living simply for decades.