It sounds like more water needs to be flushed into the tank from now on each time it's used to prevent the solids from being stuck in there.
Meanwhile, back at the ranch, letting it sit full of water should loosen things up in a couple of days. There is the old ice cube trick. Fill the tank about 1/4 full, put a bunch of ice cubes down the toilet and go for a drive with lots of curves, corners, stopping and starting that will cause the ice cubes to slosh of the solids to wear them out of there - the ice cubes sort of serve as sandpaper. The more ice the better - several bags as it melts quickly.
And leave some water in the tank (when it's not freezing out) to keep any left overs from drying out into solids.
Bill
