We have a 2017 Bighorn 3270. When the black tank is full, we get a leak on top of the tank where the toilet pipe fitting enters the tank. It is under the fitting that the ABS pipe attaches too, not the ABS pipe. We have tried all types of sealants,tapes,glues and caulks. We can find nothing that adheres to the LDPE (Low Density Polyethylene) that the tank and its fitting are made of. How does that fitting attach to the tank?