Vehicles that get good mileage are going to be lighter duty so when you take them off road your chances of doing damage increases a great deal, so I agree with everyone else that says that a $500 off road beater is the way to go even if you have to pay to store it, the increase in gas mileage of a road only vehicle is going to offset the increased cost of owning a 2nd vehicle.
If you really do want to own just one vehicle look at the Suzuki Samuri/geo Tracker.
You can also look at the
Search by Class for Fuel Efficient Cars
and look at ages of vehicles that fits your price range.