I'm not sure what you meant by "sewer line water" but if that's the line for the black tank flush then that would be the cause of the water overflowing from the toilet. That line should only be connect when you are dumping the black tank and the black tank dump value is opened.
If the connection you are referring to is the standard city water connection then there has to be a leak in the toilet flush valve that allowed water to fill the toilet bowl and overflow into the trailer. The only entrances for fresh water to get into the black tank are via the black water flush or operating the toilet.