Oh no! The dreaded "Poo Pyramid of DOOM!" My step brother had to deal with this on a 5th wheel he bought last year. Worst part was it wasn't his! And it was quite "established"!
As has been suggested, use plenty of water. As for getting rid of the existing problem, I would suggest filling the tank up as much as possible and letting it sit for as long as possible. If you can add "Happy Camper" I would try that, or at least some chems for this process. Letting it sit will allow things to break down some and soften up so the tank can be flushed properly. Water softener (I use Calgon liquid softener because that's all I can get) will also help things "slide" out easier.
And then for the future, again, the other posts have it right on: make sure there is water in the tank before you use it, use plenty of water when flushing, and don't dump until you're almost full. As for what toilet paper to use? I use the Angel Soft recommended by "The Fit RV". Never had any problems in almost 3 years so far. But then our longest outing has been 3 nights, (maybe 4) so the paper doesn't really break down all that much anyway. But it's a lot better than what we use at home which doesn't want to break down much at all (it's good stuff!) We depend on LOTS of water and a good tank flush at the end of the weekend. So far, that has served us VERY well.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!