For smaller cracks and repairs, vinyl repair (home depot or ace) has worked well. It seems that sunshine over time tends to brake down the screen. Using this with toothpicks etc a fine line can be created that bonds nicely. The window material can fail on the stitching in the corners probably due to the weakening of the material by stitching and corner stresses. The vinyl repair works very well (invisibly) for that. It has lasted for years without sign of weakening.
I suspect Sportsmobile has the top warrantied, not necessarily the windows/screens.
The other item that helps windows is using Aerospace 303 Protectant
http://www.firstchoicemarine.com/p-1242 ... 30340.aspx ,
a water based silicone on the plastic and zipper teeth. It took a bit of research to find what this is, and the key difference is this is water based, rather than solvent based. It is used extensively in the marine industry (West Marine, Amazon etc). It appears to make the vinyl more supple and the zips are very smooth! Freezing camping window zipping used to be a concern...