Adding portable battery capacity to a truck bed will consume payload capacity in a major way. Adding battery capacity to a trailer will likely increase the tongue weight, also affecting payload, in addition to less stuff which you can put in the trailer. In both cases, adding $20k in batteries which will also degrade with time, will require replacement, and won’t be used daily makes little sense to me.
A diesel generator in the bed to provide extra energy makes a lot more sense than batteries, and weighs less, however that with the fuel will also decrease available payload, and needs to be factored in. You’ll also need to be able to get it in and out.
But the best solution is to simply choose the right tool for your job. If you do long distance towing, just buy a gas or diesel truck. Multi-car families can own the best of both worlds.