I do something similar, I pump into a 55gallon barrel with a flojet. The pump has a check valve, so once you shut the pump off, it doesn't come back thru. I then have a ball valve on the barrel that I shut off, then when I am ready to dump, I disconnect the outlet hose from the flojet ( the ball valve being shut off on the barrel holds the water inside the hose) and stick that end in the ground, then turn on the ball valve and drain away. I do both black and grey, never had an issue with getting messy. On a long weekend, I will pump over 300 gallons between my trailer and other family. It's a great set up