If it's leaking...
Sometimes it's not the gasket, it's a tiny crack where the tank-flange mounts. Is your black tank polyethylene (Thetford) or is it ABS(unknown manufacturer)?? Cracks in polyethylene are best welded
. Cracks in ABS are best epoxied
Gaskets come with a new Thetford valve.
!968 was an early, somewhat experimental black tank era. For a 30 foot trailer, It would be reasonable to reconfigure the waste system with modern grey and black tanks using Valterra valves.
If time and $$ aren't easy right now, the link below has possibilities...