Two other things you could try:
Depending on how the tank drain is arranged, you may be able to snake a hose or piece of tubing from the drain up into the tank. Squirting around inside the tank might help break things up.
Adding a bottle of Liquid Plumber or Liquid Draino to the tank, filling it up, and letting it soak could help. The caustic lye in the drain cleaner should help things dissolve.
Bill & Kate - Stone Harbor, NJ
w/ Bailey (standard poodle) and Zeke (partipoodle)- both rescues
2018 Ford F-250 Super Duty Crew Cab w/ 6.2L gasser
2014 Forest River Wildcat 272RLX fifth wheel