I always cover my van from October to April. I think its wise as I have an elderly caravan and think it helps keep it in good condition. I have a Maypole cover that I've used for the last 6 years with no problems. I also put it on axle stands to save the tyres getting flat spots while stood.
Having had our previous van for a long time (15 years) there is no doubt in my mind that the van front panel in particular does suffer with time so is better covered.
The biggest issue to me is not whether they cause condensation, or putting the thing on, but taking it off. There is one heck of a lot of material in covers, and it impossible not to have it on the ground when trying to fold it up, This is when the risk of getting dust and dirt on the inside appears.
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