There is no contradiction. The "key words" here are: "neutral nor ground".
This is in reference to hooking up your trailer to an AC power source -
*NOT* the 12vdc power battery source.
The subject being discussed is AC current, *not* DC current. That is the
difference. IF you were using 12vdc current you would then want to ground
the negative side ( - ) to the frame or body of the trailer.
However, with 120vac the ground needs to come into and then out of the
trailer *back to the AC source* - for safety - to keep any accidental
discharge from killing you or someone else.
Tom
WBCCI 5303