Who gave you the estimate? Seems that they are planning to replace more than just the roof and roof plywood. Do you have any of the new spray-on roof repair facilities anywhere nearby? RV Armor? (
www.rv-armor.com). If so, they usually can take the membrane off, repair rotten wood and spray a lifetime warranty roof on.
Here is an article on a MH roof replacement by a dealer in OR...
http://www.motorhome.com/rv-how-to/r...f-replacement/
Any chance of insurance helping out?
Joe