Black water valve leaking, NOT a simple valve replacement

Mark,
Bladex valves are a simple change out. Do you see a leak on the flange to pipe it bolts to? If so, grind the area clean with such as a Dremel, wash with acetone, repair with pc 7 or 11 epoxy putty. Every good hardware store has it. Works wonders on pvc/ cpvc/ copper pipe.
Pipe to tank connection requires welding, as polyethylene will not glue. ( you know that)
You can carefully cut off the 4 bolt flange with a dremel and propane torch and glue on a new one.
Max