You can get emzine cleaners from places like Home Depot or Lowes. Pour some down your drain, and it eats the gunk in your drain lines. High rise apartments and office complexes use these to keep pipes clean.
Put some in your drain with very little water in your holding tank and let them eat the grease. Its a start.
FYI, we ALWAYS wipe all our pots, pans and dishes with paper towels to remove the grease (as much as possible) before washing in the camper. At home, we are on a septic tank, and do the same also. NEVER put the grease down the drains. Use paper towel and throw it away.
