The requirements for decommissioning a sewage system will depend on the type of system you have and the materials that were used. To decommission or relocate an outhouse, you will simply need to remove the above-ground structure and then backfill the hole up to normal ground level. It is a good idea when digging a new outhouse hole to leave the earth piled nearby to be used when you need to backfill in the future. If the above ground structure is not going to be used in another location, dispose of it in the same manner as any other building, by removing the material from the park.
If you have a septic tank and weeping bed, then decommissioning will depend on the material that the tank is made from. If the tank is concrete, then the tank should be fully pumped out by an approved sewage pumping service. At that point, you can either remove the tank or the tank can then be crushed and buried where it is (if it is a concrete tank). If the septic tank is plastic or metal, then the tank must be pumped and then removed from the park and the hole backfilled. The tiles in the weeping bed may be left in the ground where they are if they are clay tile. If they are plastic tile they will have to be removed.