Black water Tanks

[Kpayneps]

I have a 2018 3761FL with two washrooms. I appear to have a two black water and two grey water tanks. I was flushing them out and put water in the small washroom tank and then went to fill the rear tank. When the rear tank was getting full the toilet in the small washroom overflowed.

The questions is are they not separate tanks or are they linked someway that when one is full it overflows to the other?

Thanks

[Camp CA]

The specs say 39-gallon black water tank capacity but does not say whether it has 2 black tanks. To confirm, how many "black" tank dump valves and flush connections are there on your trailer? If you have one (1) then then I assume the tanks are connected or you have one tank.

[rohrmann]

It’s very likely your rear waste tank serves the galley as well as the rear toilet. Not sure why you would intentionally fill a waste tank, but now you know not to do that.