I know some people who like to put their bike in their tent in case of bad weather, but I have always just left it out, locked up if needed. Usually I have a pieces of plastic to wrap around my saddle. If it rains and it is windy, that's life. The bike gets wet. You wipe it off and oil the chain, and you go. It's not any different than riding through a rain storm. The bike still gets wet.
For something between your sleeping bag and the ground, I would suggest buying a Thermorest pad. It rolls up tight during the day, and takes little space. And I have found them to be comfortable. In my younger days, I used a foam pad, which also seemed to work, but is not as comfortable.
Dave