Not meaning to detract from your very clear question - as someone who works with winches regularly, I suggest that for use on a trailer, any of the commercial solutions (which you can easily get up to the 17,500 or even 19,000 lbs rating) is a better choice than a mechanical truck winch off a 5ton, converted to electrical.
Consider that, for most use cases, you will pull the load onto the trailer from more or less right behind the trailer. That leaves you with enough winch cable to use a snatch block, double the line and double your pulling power, if you need to. You may therefore be able to use a lighter rated (and cheaper) winch on the trailer to do the job.
If really toying with the winch off a 5ton, it would perhaps be more straightforward to drive the mechanical winch with an hydraulic motor and use an electric setup to run that. I suspect that, by the time you have cobbled that together and priced out the components, a complete electric winch will likely look like a sensible deal!