That's a good idea. I tried it once when my previous car, a Taurus, broke down while towing the boat. What I found out was that no rental company in NY will allow you to tow anything with their vehicles unless they provide the trailer. Not even U-Haul. The only way you can tow is if they provide the "trailer", or the dolly for hauling a car.
It's a bad idea in terms of fuel use for me, though. I work at home. Unless I'm towing the boat any distance, I put very few miles on the truck. If I rented, I'd be using the same amount of fuel (or more) in a similar rented vehicle.