I will add to the comments here with a solution that has worked for me in the past to dried clear debris in the tanks. As long as the valve is opening, then I suggest trying Thetford Holding Tank cleaner called Tank Blaster. It is designed to clean sensors, but seems to clean my tanks too.
https://m.media-amazon.com/images/I/...,0,0_SH20_.jpg
Easy to use, fill the tanks with water (sounds like this has been done) and then add the appropriate amount of chemical. Let is sit for 36 to 48 hours. The chemical and water should dissolve the poop pyramid or at least break it up...
Best of luck...