As just about all of us have read here, certain models of WW trailers have defects that have resulted in a number of different problems. To one degree or another, this subject has come up on this for...
With as long and small as those roof supports are I doubt you could ever get all the sag out even if they were brand new Oak. You might think about reinforcing them with a piece of angle iron bolted to them.
Thats why I dont like getting on top of my trailer. There just isnt much support up there.