I suspect you have the same trailer as me. I've never had to do much to protect the top, but my trailer has always been under some kind of cover for most of the time I've had it. (I bought it new in 2007). You could certainly apply wax to the top, or, if mine ever shows signs of degrading, I plan to coat it with two coats of white elastomeric paint.
I DID apply that paint to the upper and side surfaces of the rolled up awning, which is exposed to the sun when the awning is retracted. This protects that area from the sun's UV radiation which will degrade most fabrics.
The fabric cover I have now is from CalMark and is made of Sunbrella. Here's a link to the thread where I show it.
http://www.hilotrailerforum.com/f32/...er-cover-5677/ My old Sunbrella cover, which was warranted for 5 years lasted 10. This one has a 6-year warranty.
I also have it under a "carport" style shelter that I built which gives it additional protection and keeps it cooler in the Arizona sun. The shelter also keeps most of the rain (and snow) off too. (I had a 30"+ snowfall last winter).
The sun, I believe, is your major enemy though. Anything that keeps it off reduces the degradation that opens holes for water entry.
- Jack
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