Yes I did give the LL a chance to clean it out. He kept saying that he would get to it, but never did. And since it was backing up into the house, we got it done. And the tank was completely full.
because it wasn't pumped for the seven years your contract was in force.
You did not specifically state the problems arose prior to the expiration of the contract but it appears it could be so based on this:
As of April 1, 2012 the contract became void because we could not come an agreement with the owner
Our sewage kept backing up, and we finally had a company come out and pump the tank out just last month
If the back up started prior to the expiration of the contract, due to this:
In the original contract it said that we were responsible for all maintenance & repairs.
You are liable. Just the same if the roof needed to be repaired prior to the expiration of the contract and you failed to repair it or any other repair you failed to perform while under contract, you are now liable for the cost to repair.
If the back up did not start until after the contract expired, you might have an arguable point the work did not need to be done until the contract expired. I would suggest that you would likely lose even that argument though since this type of thing does not simply start overnight. There was likely symptoms long before it was obviously the problem. That makes it your liability to pay for the work.