If you have one that is surrounded by fields (and only a single road tile), I’d suggest to take that one.
You can use the Cut logs or Clear a forest jobs in the camp to free up surrounding forest and - to my knowledge - swamp overmap tiles, but it will take some time.
I might be mistaken, but I don’t think you can expand onto a road overmap tile (as it only seems to allow field tiles) and you can’t clean it up. So if there are more than 1 road overmap tiles adjacent (diagonal directions included), you will not be able to build all the camp extensions.