The most common issue is when the wind, rain and snow blow the cover around and 1) wear the fiberglass at contact points and 2) trap moisture under the cover which causes oxidation and is bad for metal parts.
Covers are a mixed bag. New RVers think (and are told by sellers) "I must cover this investment." But go to any RV storage lot and look at the number of covered and the number of uncovered. It's probably 10 uncovered to 1 covered.
Also, covers tend to last only 1 to 3 years before ripping to shreds.
Do a search on "Covers" on www.irv2.com
. You'll find a variety of opinions on the subject there.
If the tress on the property are likely to drip sap in the spring on the TT roof, then you may want to cover it as that could be bad.