A bike cover will just hold any condensation, would look at rust inhibitors, like Boeshield or ACF-50 (think Aviation corrosion prevention) for inside the frame, keep the rest of the bike well lubed, but accept that you will get rust on hex heads & the steel parts of the drivetrain eventually.

Some brands are worse for corrosion than others, have found that any steel parts on Avid brakes will corrode within a year or so, what ever condition that are kept in, but Shimano steel brake parts kept in exactly the same conditions will stay corrosion free for many years.