Believe it or not mpg is not drastically changed when towing. The main cost is the set up. The peace of mind and convenience of having your own toad is (as they say) "priceless".
You can give it a try to find out if it works for you. There are a lot of things that can and do come up that make having your own car along vital.
Renting limits your choices for stops. Some locales do not have rental agencies. Most larger metropolitan areas will but smaller towns may not. For example--if you come to our region...a car dealership 30 miles away handles a few rentals but the nearest agency is 75 miles away. One time we went to Gatlinburg, TN toadless and found the only place to rent a car was from a towing service. The car was a stinking wreck and we had have the tow truck driver go miles through the mountains to get it and return it. The nearest agency was Knoxville, TN.
We found going without a toad was simply not safe and very inconvenient. We've had many incidences when we'd been in real trouble if we'd not had our own vehicle along. We'd rather depend on ourselves than on the availability of a rental anywhere we may happen to be.
