It sounds like they've scoped this tank a couple times, even dug up the input line for repair, but have they never opened up the inspection covers on the tank to see what's going on inside? On my tank there are 3 inspection covers. One big one over the center, and smaller ones on each end, covering the inlet and outlet. I'd expect a visual inspection sometime in all of this. How do they know that the tank has settled on one end, and not that your leach field is clogging up, or that there's solids blocking the outlet partially, so that the water level in the tank is a couple inches higher than usual. This sounds like a strange inspection method to me. Tell them to get out a shovel and get their hands "dirty".