Blackwater tank

Add a generous amount of some kind of water softener and fill the tank about half full. Let it set for a day or so and then take it for a drive before empting the tank. Then use a black tank water wand and flush....flush and flush until you see clean water coming out. Messy job, I know.

Ouch! I saw a video showing that the bag of ice trick has a very limited effect on the solids.

You might want to try filling the tank with water for a while to try to soften up the solids. If you have a built in tank sprayer try that. If not, the wands you stick down the toilet work pretty well. Kind of "power washes" the solids.

You need some moisture to soften it up. High pressure nozzle. Not a spray nozzle. You need some pressure to break it up. About 30 years ago I built a wand. About 30" long of 1/2" dia copper pipe. One end install a ball valve and hose fitting. The other end install a cap. In the side of the cap at the bottom drill a 3/16" hole. About 1/4" up from that hole drill the 2nd 3/16" hole. You can insert it through the toilet and rotate it around. If your black tank is really clogged it will take a while but eventually loosen and flush it out. You need to leave the black valve open when flushing.

Sorry to hear of the bad advise you were given. After dumping at campground get some bags of ice cube before you leave and dump in black tank. As you drive it may help to breakdown the hard stuff. Place plenty of water in tank when storing your unit. Others will also give info from their experience.