a grease trap is for the kitchen sink and may have it’s own drainfield, it may not be conected to the septic tank. If the problem of your tiolet not always flushing, along with the area around the septic tank always being wet, sound like you have a hydralic overload on your drainfield, too much water use. If the toilet don’t flush after several loads of laundry or several showers than you need to cut down on your water use, or get a larger drainfield