We had a block in our toilet back in February and once we got the toilet out we noticed a small hairline crack in the bowl so we replaced the toilet. I noticed a month later that when the tub was drained the toilet bubbled. In the past few months now with the bubble is an awful sewer smell. It started occasionally and now is getting now so that every time we take a bath the bathroom stinks from the toilet bubbling. There is no rise in water in the toilet. We are on a septic system. In February when we had the partial block in the toilet I climbed up on the roof and put a rod down the vent and it was clear.

We live in a tiny cabin and only have 1 toilet, the tub and a kitchen sink on the sewer line. They all that the same vent. Since replacing the toilet we've had no other problem like we had been having with the tub draining slowly and the toilet filling to the top and taking forever to drain out. The sewer line was snaked when the toilet was replaced just to make sure it wasn't blocked anywhere else. The system is 12 years old.

What could it be and what can I do to fix it?

I noticed a month later that when the tub was drained the toilet bubbled.
This indicates a partial blockage downstrem from the tub. What's happening is thawhen you drain the discharge hits the black each and bounces back sending a bubble of sewer gas ahead of it Hence the spell and the bubble. The solution is would be to pull the toilet and snake from their.
Now I am confused. You say
Since replacing the toilet we've had no other problem so the problem is fixed? Back to you, Tom

I said "Since replacing the toilet we've had no other problem like we had been having with the tub draining slowly and the toilet filling to the top and taking forever to drain out."

This is what the old toilet was doing before replacing it in February.

Since replacing the old toilet in March I noticed it start bubbling when draining the tub. In May and June we started getting small amounts of sewer smell and it's progressively been getting worse.

Ya got me!

I said "Since replacing the toilet we've had no other problem like we had been having with the tub draining slowly and the toilet filling to the top and taking forever to drain out."
So that wasn't the original complaint? You were simply telling us what the old toilet did?
So why are we here? Please leave out the history and if you have a current complaint let us have it. Back to you. Tom