First, what year, and model of Land Yacht are you looking at? Second: how do you know that the roof is leaking?
Basically, any Airstream has thousands of rivets, many seams, and several penetrations (doors, windows, hatches, vents, chimney, etc.). These are all opportunities for leaks.
If you are convinced the leak is coming from above (ie., there is a rotten spot dead in the middle of the floor), then it might narrow down the area you need to look at. Any weekend warrior can clean and reseal a seam, but it could be leaking screws/rivets/gaskets that will take a bit more expertise.