JoeyDrunk - Let's try something. Want to be our guinea pig?
You say you're not there often. Can you go 3 or 4 days without dumping the black tank? A week?
Dump the tank, pour a generous amount Rid-X into the tank and leave it in there as long as you can before dumping and repeat. Use the tank as normal as you do this. Let's see if it dissolves your mound over time. Give it 3 weeks to see if it starts to clear things up.
No mess, no cramming a broom handle down the hole risking the chance of punching a hole in your tank, no endless hours of spraying a wand. Just pour and forget it.
Come back and let us know if this solved your problem.
On Edit: Rid-X works better in warm water. Are you in a location where it gets up to at least 60 degrees during the day?
