We have an '18 Toyota Highlander with the v6. There's nothing in the manual about what gear to use while towing. It does have what they call the PWR mode. Near as I can tell, it adds power to the engine but has nothing to do with the tranny. I asked the local dealer what gear was recommended while towing and he said to just use the PWR mode. It also has an alternate shift mode that limits how high of a gear you can be in. It also doesn't mention a torque converter. I'm still dubious. I can't find anything on the web about it.
PWR mode should hold each gear longer (higher RPM shifts) and downshifts @ higher RPM as well. It keeps you in the upper end of the RPM range nearer peak HP/TQ. I suppose w/variable valve timing there could be algorithms written/engaged to skew towards power rather than MPG as well.
