> Charging efficiency isn't 100%, so you'd have to tow it further than the range you need

That's not how it works. You basically get 5 mpg or less on the truck, while the car charges at significantly higher rates than the speed of the truck (in mph), because the car is more efficient at using the energy.

https://insideevs.com/news/514727/tesla-towing-70mph-fast-ch...

Towing at 70mph in the above example was charging the car at 65 kW. That's equivalent to ~260-270 mi/hr of charging rate assuming 250Wh/mi.