TRUCKEE — Conservation groups Friday morning announced the purchase and protection from development of the nation’s largest cross-country ski area, the 3,000-acre Royal Gorge property on Donner Summit in the Sierra Nevada. The Truckee Donner Land Trust and The Trust for Public Land, working as part of the Northern Sierra Partnership, have raised $11.25 million, meeting a Dec. 20 deadline, to complete a deal that was first announced in early August. The total includes private donations along with public money. The groups bought the property from court-appointed Receiver Douglas P. Wilson, who took over the land after a failed development …