If you have a roof tent then I would advise you to stay in the middle of Swakopmund unless you want to be putting your tent up and taking it down every time you want to use the car.
There are two camp sites in the middle of Swakopmund, both very close to each other; Alter Bruecke and the camp site by Tiger Reef Beach Bar. Both are only a few minutes' walk from the middle of town, near the aquarium. Alte Bruecke is not a soulful place. Rather, it is clean and functional but I prefer it because it is much more secure. The camp site by Tiger Reef seems to function as the car park for the beach bar so it gets a) full of cars and b) very noisy at night. It is also not at all secure, although it is quite cheap. I sleep like the dead so the noise of car engines and doors slamming for half the night didn't bother me too much but my wife hardly slept at all. Alte Bruecke is more expensive but it is much more secure and quieter too.
Lastly, all the stories you have heard about Swakopmund being cold at night are true. if you are in a roof tent, make sure you have plenty of blankets or sweaters. You may well need them. Even in December, it may not be that hot at night. I was there in November once and it was still bloody freezing in a roof tent. As the guide books suggest, even if you're a happy camper, Swakopmund is one place where you really want to have a think about springing for a room somewhere if you can afford it.
