I'm not sure why you want access, and you might be allowing some pretty foul odors to float out. Digging one up isn't a big deal, they aren't normally more than a foot or so deep.
I have never seen a cast iron tank, I have seen plastic, concrete and even 220 gallon oil drums used as tanks. I used a 55 gallon drum once at a hunting camp.
I agree with John, have it pumped, inspected and repaired if it seems dangerous.