What those both (Toad Charge and Towed Battery Charger) appear to be is a 10 amp DC-DC battery charger, where the built in charger controller will step-up the available voltage from the towing vehicle (drawing more amperage in the process) to a voltage that will do a much better job of charging the battery in the towed vehicle.
If these units are any good, the charge controller should be at least a three-stage charger (bulk, absorption, and float).
If that’s what they do, then they would provide better battery charging than simply connecting the towed vehicle’s battery to the towing vehicle’s battery.
:):)