Murfman

Hello, my wife and I own a 2018 - 30' Keystone Bullet UL travel trailer. This is our first RV and we purchased it last year April 2021 and have cleaned and painted it, repaired and/or replaced many parts. It is in very good condition but on our last trip the black tank rinse port where a water hose connects stopped allowing water to enter the black tank to rinse. I've checked the line from the outside port all the way to the tank. I replaced the Backflow Preventor under the sink. I can get water all the way to the tank from the outside hose connection to the tank, the entire water line is clear and flows. I've taking the 90 off from the black tank and there is flexible rubber (feels like) flap blocking the hole in the black tank. I cannot push it in or move it. I assume this is my problem to getting water into the black tank to rinse it out after dumping. It has worked fine for the last year and 1/2 up until last week. Now water will not go into the tank. Any suggestions??? Thanks!!!