how to repair woodrot dampness in a caravan?

Bought a caravan with dampness and the front corner and back is now rotten due to a leak from the roof (I think). To have it repaired is very costly and my husband would like to repair it himself. What advise could you give us. Where to buy materials needed, what adhesives to use. Is it advisable to use masonite covered with the necessary material.
Thanks

Angela
September 2011