The best way to protect an air conditioner shroud is to paint it. The plastic used to form an A/C shroud will degrade in sunlight and become brittle. Painting the shroud protects it from UV light both when the trailer is stored and when it is in use. Unlike a fabric cover that only protects against UV when the unit is stored.
The only benefit to a fabric shroud cover is to keep dead leaves, bugs, and other detritus out of the air conditioner. But if you're going to get up on your roof to apply or remove a fabric cover, you could get up on the roof to clean out the shroud instead so that detritus doesn't build up in the first place.
I thought getting old would take longer!