There are 4 bolts that hold in each valve blade. Check around those bolts. The sewage lines coming down to the main opening are suspended and where those bolts go thru the plastic take a beating going down the road.
I thought I needed a a new valve blade because it was dripping a little leak. When mobile RV came out to replace them he showed me where the bolts were, the plastic was cracked. We looked at the other side and those too had cracks around the bolt. That's were the small leak was coming from.
When we went to loosen the bolts to get the blade out the plastic around the bolt actually broke off. Had to replace the whole lines.