the bear II wrote:
It should empty completely....every tank I've ever seen the bottom slopes towards the dump valve opening. There may be some residual liquid like a cup or so but nothing like you are describing.
Sounds like you might have the "Pyramid of Death". A pile of solids at the bottom.
You can try dumping a couple of bags of ice in the tank and then drive around to see if it will break up any solids.
If that doesn't work then it's time for a camera to see what might be stopping it up.
X2, then maybe a good power flush through the toilet to flush the solids that have been left from the previous owner not dumping properly. JMO