Winter storage
If your going to park the car on dirt than there's not much you can do to prevent dirt from splashing on the paint. I wouldn't want my car sitting in dirt for a few months anyhow.
Can't you park it in a driveway or on the street?
Some interior ideas. Put some kitty litter or speedy dry onto a paper plate and place it somewhere inside the vehicle. It will trap any moisture you have floating around inside the car and keep things like mold from apearing on you seats etc...
You can put another plate of it underneath the car as it tends to keep some rodents out as well.

