Yes and no. Cost of fuel is also a big factor. A big rig is typically running about 6 MPG, and for lighter loads that doesn't change very much if you're hauling a half-size or full-size container. Sending two trucks means burning twice as much fuel, which could easily be hundreds of dollars.
But definitely that's less of a factor with an electric truck since the charging cost will be a lot lower than a comparable amount of fuel.
I think the bigger savings will come in being able to precisely control stops, starts, and turning, which will both minimize damage to the road surface and lengthen tire life.
ISTR a case where a large open pit mine bought GPS-driven trucks and saved a great deal just from longer tire life (each tire was $20K).
Re: gearbox, I wonder whether an autonomous truck could be paired with a little scouting drone to "look ahead" at things like blind corners or stop-and-go traffic, to allow the truck enough foreknowledge to gear up and down more gradually.
But definitely that's less of a factor with an electric truck since the charging cost will be a lot lower than a comparable amount of fuel.