Water, lots & lots of water, and a digester. You might want to cut the end off a garden hose and use it to break up the blockage.
If it's a straight drop into the tank a cleaning wand will work even better.
Get some rubber gloves and a change of clothes. There's a good chance the clog will also block your tank drain. One of these will help with that. Flush King - Valterra FK-01 - Sewer Flushing - Camping World
Google 'the geo method' for other ways of dealing with it.
It's a crappy job but someone has to do it.