My guess, and I better not have this again on this rig!
My last camper, the cut out for the tank was still in the tank!
I would drain the black tank and it would flow full force then all of a sudden it would just trickle out, lived with it for years just thought that is how it is supposed to go. I did a fill and flush of the black tank a few times to clean it out really good, I was looking down the toilet to see the level and that it was cleaned out. I saw a piece of plastic float by, the size of the cutout. I never was able to get the toilet off to remove it.
I sure hope Forest River does not leave those in there too.
2016 Grey Wolf 26RR Limited
92 GMC K2500 454
2006 Honda Rubicon ATV