Open the drain valves and the stuff will flow......tanks are higher than cg hookup.
Dump black tank first then dump grey water tank.
Then when done dumping you will have to 'walk' the sewer hose to drain it...probably a few times.
Leave grey water tank drain valve open so you have the vent on tank help move stuff in hose along as you walk it.
Just one of those things you have to deal with..........but ain't camping FUN :B
Course you could buy a portable macerator pump and pump your tanks out.......$$$ and much slower but you wouldn't have to walk the slinky.