The issue is that while trying to conserve water and conserve space in your black tank, you are creating the mound due to a lack of water. Gravity alone is not going to get the paper into the tank if their isn't sufficient water to propel and lubricate the waste as it tries to slide down the chute. This is not waste jumping up from the tank, it's waste that never made it down.
So, the solution is to poke the mess with a stick and get it into the tank; or pull the mess out and discard it, or pour a bucket of water down the chute with sufficient volume to dislodge the mess and send it into the black tank where it belongs. Once you have the immediate issue resolved, prevent it by not being so stingy with the water you use to flush.
As a side note, if you're too conservative with the waste water, you run the possibility that when you are emptying the tank, you may be leaving behind some waste mountains that are not being flushed out due to a lack of water.