My friends spent 6 months living in a static caravan in their own front garden after their house had been flooded.
It was hell. Although it was plumbed in to the mains drainage, it was a very cold winter and the waste pipes froze solid. Their calor gas froze, too, so they had no heating, cooking or hot water for some time (they came and stayed with us). They had mains electricity though, which was a positive.
Their daughter was 17/18, so doing her a-levels, and had no space to work (a disadvantage when one of them is art). She also had nowhere to spend time with her friends, as the second bedroom was tiny.
They put on loads of weight, because they weren't going up and down stairs several times a day, and because they were quite limited in what they could cook. If you used the oven or grill, the gas pressure to the burners was so low it took an hour to boil potatoes. It was really hard to keep tidy, because storage space was so limited, and they all got on each other's nerves.
It was a really grim, distressing and frustrating time. However, they were in their early fifties, so perhaps less adaptable than a younger couple, and they were already very stressed because of being flooded out and losing almost everything from the ground floor of their house, and all the hassle with the insurers etc. I think it may well be easier for a younger couple who are doing it for a reason.
Two youngsters in a confined space would be a challenge imo!