Towing a car will not affect milage measurably. And if you use the car for a few trips to the grocery store or whatever instead of the bus, you will be fuel ahead.

On our last 4 week trip we put 2800 miles on the bus and 3500 miles on the car separately. Just running around. We normally only stayed in one place for a couple nights.

The fuel should not be the issue, the convenience should be.