Could you explain your definition of a "true cost/benefit analysis"?
Yes - apologies for not being clear.
When analyzing the financial impact of changes in energy use (such as replacing incandescent bulbs with LEDs), it is common to perform a cost/benefit analysis from several different perspectives. Typical perspectives are the financial impact on the participant (e.g. the home owner), the utility (having to deliver less electricity), and “society” in general (e.g. impact on electric rates and emissions).
Each of these perspectives is impacted differently from the action to use less energy.
If you add the trailer and its electrical system to the cab, and you want to know its full impact on energy usage, you need to consider the energy cost of manufacturing the trailer as well as energy savings reaped during its operation (if any).
At first blush, that trailer looks like a good idea for the user, but, as you said, you need to know the operational cost vs fuel savings, and translate it all into dollars to know if it is cost effective or not.
This document is a bit long in the tooth but illustrates some of the concepts: