Grey water system to black water holding tank

[Raider]

Hello,
New to the forum and new to class C motor homes. I recently purchased a 1985 Jayco 26 Jayco mini. It has not been used since last year and was still winterized. Today my father and I started getting all the systems ready to use. We hooked up city water to flush the systems. Soon after I discovered a leak on the black water tank? That was a surprise, I was checking grey water systems and find I am getting water in the black water tank? Is this normal? We did not flush the toilet before the leak. We traced all the grey water lines to no avail. Looking under the camper I could not see any lines that someone ran to the sewage tank.
Your thoughts would be very much appreciated.
Thanks
Greg

[Raider]

I need to check the flusher on the toilet for leak by.....that has to be the only way the 2 systems are tied together. Does that make sense?
Greg