We are on a holding tank and I have been battling a fowl smell coming from my basement. It smells like sewage. My whole house smells of it and it is really disgusting. We have a whole in our basement floor right next to the furnace and I believe that is a run off for the a/c. We have covered that up thinking that would help not working.We have checked all our vents and nothing is clogged. What to do?

Your post is confusing.
We are on a holding tank What does that mean? A open vat holding water for your house? A 40 gallon galvanized tank?

We have a whole in our basement floor right next to the furnace and I believe that is a run off for the a/c So what smells? The holding tank or the hole for the AC?
Care to repost you complaint again? Back to you, Tom