Black Tank Issues

When we purchased our used 2012 Montana 3750FL, DW and I took it out on a shakedown cruise. There were a couple of issues that were easily fixed, so no further problems there. But when I emptied the black tank, I noticed there was still fluid and some solids still in the bottom of the tank. I flushed the tank several times using the built in tank flusher. Weak flow by the way. Even after I stuck a hose down the toilet. I even filled it with a 5 gallon bucket to verify the capacity. There's always fluid and gunk left over. I can't seem to get everything out. You may find this gross, but I even taped my iPhone to a yard stick, turned on the video camera, and stuck it down the toilet to see what was down there.

So after reviewing the tape, I noticed that the tank empties at the side, right below the toilet. And right in front of the hole, there seems to be a bulge in the tank. Right below the toilet where everything drops. It's about 1/4 inch below the level of the drain pipe. The rest of the tank drains just fine, but there's always this large puddle of nastiness left over.

Just to be clear, my intent is to NOT leave the tank completely dry. I generally pour in a five gallon bucket with RV RidX and Calgon after I flush the tank. But I'd love to be able to get all the nastiness out before I do that.

Is this an inherent problem with all black tanks, or is it just mine? How do I get rid of the bulge/puddle? Is this something I need to take to a dealer to fix, I can fix myself, or am I just up a puddle without a paddle and have to live with it? Thanks in advance for any help.