I have a Flojet for dumping at home in the sewer clean out. Works fine but if you follow the instructions it is a pain to use and I only use it when a slinky hose is too short. They say to fill the gray and black tanks at least once with fresh water and pump them out to remove everything. They say this is necessary since you don't get the big rush like you do when dumping through a 3 inch hose.
The following is from the Flojet manual:
1. Remove end cap from drain pipe.
2. Install portable waste pump on drain in any
convenient position, twisting clockwise to lock.
Be sure that o-ring is on pump adapter prior to
installing unit.
3. Install waste/garden hose on discharge port
adapter.
4. Install rinse water hose to on-off valve.
5. Open black water valve, turn on pump.
6. Turn off pump and open gray water valve. Let gray
water backflush black water tank. Turn on pump.
7. Turn off pump, open rinsing valve and back fill
tanks for rinsing. Turn on pump. Turn off rinse
water. Drain tanks.
8. Close both drain valves, turn on rinse water (40 PSI
max.) and pump. Rinse pump and discharge hose.
9. Turn off water and pump, disconnect hose and
pump. Store until next use.
10. Replace end cap on drain pipe.