The only thing I can think of is that the pipe from your toilet bowl to the holding tank is mis-aligned or somehow not connected.
With a straight line to the holding tank that can't be more than 12 inches long, can't you just scoop out whatever is in there and take a look. Perhaps you have a rock solid pyramid extending all the way up to your toilet bowl. Your black tank rinser will likely have no impact on that at all. You said you were new to Rving, but didn't say if you have a brand new unit, or if you bought it used from someone. If it is used and they never properly used the toilet (didn't use a lot of water when flushing), then the poo pyramid is possible. Again, scoop out what you can from the toilet and pipe and then get a poker and see if you can break up the solid hard pile.