In article q3Slc.11830$k24.5661@fed1read01, Jay Beckman
wrote:

Could moisture/mildew be a problem with a full cover? Or is there some way
for air to circulate enough to mitigate such problems?

Trapped moisture is DEFINITELY a problem and could cause serious
corrosion and/or rotting.
The 10 year figure is due to ultraviolet breakdown.
Depending on the structure (wood, metal, wood/metal), you would want to
do a recover just to check out the condition.