This isn't intended to be an anti cover lecture, as I think there are pros and cons to them. The decision also comes down to where you live and what climate you're in.
With that said, after 10+ years of owning travel trailers and now a new 5th wheel, my personal belief is that in many cases, you're better off not using a cover at all. Wind can cause rubbing against the roof and fiberglass, there can be condensation accumulation and excess mold, and they're not the easiest darn things to work with.
I live in the Pacific NW, so we get a lot of rain and some occasional snow and freezing temperatures. I keep my roof seals in good condition, and I haven't had any problems as a result of exposure to the elements. The RV techs I've worked with in the past, including a couple of dealers who sell covers, advised me NOT to use one.
Again, I may feel differently if I lived in a baking hot climate where UV exposure is a concern, but all in all, for the vast majority of people, I don't think covers are necessary. That's especially true when you factor in that most people who own TTs and 5th wheels tend to upgrade them every 5-10 years.
2018 Jayco Eagle HT 29.5BHDS
2017 Ford F-350 CC 6.2L