I bought a 25m one. I then cut it into a 8.3m and a 16.7m one. I bought a male and a female plug to create two leads and this gives me total flexibility. Both leads live in the van but I have never needed the 16.7 one let alone the "widowmaker" by connecting them together to make 25m. 25m is far, far, far too long in most circumstances
The Cali will work on batteries quite happily for a couple of days anyway. I have never ran out of battery power in the Cali as I rarely stop anywhere for more than three nights.
In the politically correct age, where domestic risk is unacceptable to our controlling classes in Westminster and Brussels, you must be aware that electricity can kill. Accordingly undertaking the above work the above without proper training, audit, compliance and registration procedures can have ramifications. It is fine to start illegal wars, leading to the deaths of hundreds of thousands of people but god forbid if you touch your own electrical circuits without full state sanction.