Holy 1800 Wh/ Mi, and that is only towing 5K lbs, what about towing 15K lbs as my pickup does regularly?
One would want to make sure the horses are actually in there, providing some sort of transportation once the battery runs out.
Or perhaps the trailer could be full of Tesla Powerwalls and hooked to both solar and wind turbine to keep them full. (but the horses are sure a great back-up plan)