There are many different types of RV roofs. They can vary in material and support systems. 
Some RV roofs can be patched with the original materials.

OSB Board

This RV roof was composed of 3/8" OSB board. The damaged area was removed and new OSB mounted on the roof supports.

    

Plywood 

7/16" Plywood

           
 
                                        

Some RV roofs are built with very little roof support and are made up of 1/4" plywood glued to styrofoam. On roofs like these we remove the rotten areas, glue in a filler patch and the apply 1/4" plywood over the top. The 1/4" plywood is screwed and glued to the filler patch for extra support.

               

           

Once the roof is reskinned with 1/4" plywood, it is a sturdy roof to walk on and has more structural integrity than the original roof.

There are ways to cut some of the expense of repairs on your RV roof. One way is to reinforce the current roof, not remove and replace the wood.