The best one I have used was a Jabsco, I can't remember the
model but it was a 2 1/2 gallon bucket with a 12 volt pump
attached to the top. It had a cable with two alligator clips to
attach to your battery. The pump will pump both ways so when you are ready to empty the bucket just turn the switch the other way. No dumping. Very clean, and reliable, cost about $ 100 got mine at West Marine.