The winch takes a certain amount of power, regardless of voltage. If you double the voltage, you half the current, but since the power loss in the cables is proportional to the square of the current, you can use smaller cables with the 24V system for a given power loss. For a given power draw from the winch, the difference in how long the batteries last while wired for 12V or 24V is negligable. You could use 2 additional batteries in series, seperate from the truck batteries, but how would you charge them? Like mentioned above, using the truck batteries would require a complicated switching system and you would have to leave one of them connected to the truck electrical system when using the winch. Seems to me it would be far better to use a 12V winch and sufficiently large cables. JMO
Jim