There are many dimensions and configurations of holding tanks and some black tanks are very shallow while being long and wide. For these, it won't take long for the pyramid of poo to block the entrance into the tank. An outlet can be in the center or at the end. Poo does not somehow "melt" and flow around inside the tank and slide towards the outlet. It just sits there in a blob. There's got to be a better way...
Ours is like that and we can't get more than 3 days if doing #1 + #2. However, if doing just #1 and using the CG facilities for #2, we've gotten 9 days and maybe could have gotten more. Major difference. With our first TT, we never, ever once had an issue with use or dumping of the black tank. Wish we had the old tank back.
I doubt that the pipe extends too far into the tank as they use a flange on top of the tank that the pipe is glued to.
Always add lots of water before each use of the tank (3-5 gal.) and make sure the tank is rinsed out as well as you can. I leave the in-tank rinser on for 5 - 10 min., then fill the tank up and dump and I repeat if needed. We are lucky that we can look down into the tank to see if it's clean. If you have a built-in rinser, the head is just below the pipe into the tanks and I doubt that the spray reaches the ends of the tank very well, if at all and there must be "stuff" always left in there.
You could try a Valterra clear backflush elbow. You also try a length of heavy rubber-walled 1/2" garden hose and push it back into the tank and rinse and also down into the toilet. A CG employee gave us a piece of hose like this when we had a problem with our tank the first time.
Our owner's manual says to use Liquid Drano to clean the black tank if needed. Have done this once before and just did it again a week ago as our tank was getting to be a problem again and was always giving false readings. We have the Horst Miracle probes and it hasn't helped with false readings.
I would suggest trying the Liquid Drano method to get the tank back to what it was like when brand new. If you do this, never dump it into a private on-site septic system as it will cause serious damage to the drainfield.
If all attempts to clean the tank just haven't worked, I'd buy one of those USB pipe inspection snake cameras like
this one on ebay and see what's going on inside (only around $10-$50). If the factory used an excessive amount of glue, you can have a bunch of "stalactites" hanging down around the bottom of the pipe which can impede the flow of #2 and TP into the tank like in photo. Easy to remove if pipe is vertical but not if there are bends into the tank.
Below is a diagram of dimensions of a 32 gallon tank. You can see how it shallow these things are relative to length and width. Outlets can be on the bottom, side or end.

