It sounds like your tank needs a good cleaning.
1. Pour at least five gallons of hot water down through the toilet opening.
2. Take the moho out for about a 45 minute drive preferable down a winding and somewhat bumpy road. You want that water sloshing around as much as possible.
3. Return and dump.
4. Pour in two gallons of cool water into the tank through the toilet opening.
5. Add a bag of ice cubes.
6. Drive the moho once again as you did in #2 above.
7. Return and dump.
8. Fill black tank 1/2 full of clean water.
9. Add a small box of Calgon.
10.Fill black tank almost full.
11.Repeat step #2 above.
12. Dump.
13.If you are using a slinky, get a clear end from Wally World so you can see the color of the "flow". Repeat steps 8 & 9 until you have clear water flowing.
To prevent "poo mountain", use plenty of water to flush. Chemicals should not be needed if your system is working correctly....remember the three important word in black tank maintenance.....water water water.....Dennis