If you insist on covering it, it's best to use breathable canvas, or some other fabric that breathes. Do take note of warning labels on plastic tarps
sold at places like Walmart- they'll say very plainly that they're not meant for longterm on-vehicle use due to potential finish damage, as mentioned above.
Also:
Do make sure to leave
windows at least slightly ajar, or you'll have horrible mold problems come spring.