A number of years back I had a similar situation. Twice. I ended up using the wand that goes down the toilet but I used my air compressor to break things up because when using the wand with water the tank and toilet would fill up with water. Using air is not fun either but it eventually worked. You may want to try to get a hold of a few bottles of RV Digest by Unique. Put a whole bottle into the tank and let sit for 24 hours. If I read right it will dissolve the hard stuff in that time. I use it in my 5er and it seems to do what it says. RV-Digest
here
