I can only go by my experience. My first TT with an A/C unit was a 1995 Terry. NEVER put a cover on it and when I sold it 22 years later the A/C unit still worked perfectly.
Living in the Pacific NW, it was exposed to lots of rain, sometimes snow, and from time to time I had to wash the cover to get the normal grime from our atmosphere off it but other than that, no issues. Never even had to replace any capacitors.
From time to time I had some Yellow Jackets build a nest inside but a squirt of Wasp and Hornet killer took care of the immediate issue and seemed to prevent recurrence for a couple years.
"A wise man can change his mind. A fool never will."
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2004 Nissan Titan