Hi, same problems in UK.
A simple and cheap solution is to place a board under the covered bike and have a 60w light bulb runnin 24/7
Make sure you're drapes reach the floor all the way round.
The heat from the lightbulb keeps the atmosphere in there just right to stop the condensed moisture sticking to the bike.
It also works in any form of plastic bag you use.
I have a H-D bag with an electric motor blowing it up, similar to the ones
Old Scratcher mentioned previously.
When the atmosphere is mega damp/fog It actually draws all that sh!t in.
I left the garage door open one evening fog came down and my bike had an opaque look to it, it was P1ss wet through.
I never use the blow up facility any more. (60 watt bulb works best)
Hope this is of use to anyone.
Best regards Stretch

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