They still have to prove that the addition could mess up the computer, which is unlikely (not impossible, but unlikely) in the case of trailer lights
and charging (presuming the charge lead is connected to the battery
and not elsewhere). Brakes
might be a different story because in some vehicles (I believe Ford, for example) there are ABS wires not to be messed with.
However, if one was truly concerned, one would get the appropriate connector and wires from the vehicle manufacturer, not from Wally or auto parts store.
In this case, the connection has already been made to the truck, so an adapter would be the easiest way to go. Personally, I would replace the connector on the trailer with a 7-way.