I don't think there'd be much actual time savings. Every time I get off the highway to get gas (which means slowing down to exit, stopping at the bottom on the exit ramp, driving to the gas station, pumping gas, getting back on the highway, etc), it ends up being close to a 15 minute stop. I'd be surprised if you can pull off, hook up a battery trailer (and pay for it), then get back on the highway in a few minutes.

Might as well just stop in for a 20 minute charge at a DC fast charger, many cars can do an 80% charge in 20 minutes.