We have a 30 year old septic system that started acting up after we had it pumped the last time.

The drains in the rest of the house work, but the toilet and tub do not. When we flush the toilet, it backs up into the tub and when we run water in the tub, it forces air into the toilet and makes it bubble. The water will eventually drain out of the tub but as soon as you try to run water in the tub or flush the toilet, it backs up again.

The kitchen sink drain and the bathroom sink drain run slow but they do run.

We have used Liquid Fire on all of the drains so I don't think it is a clog.

My question is:
Could the vent that runs to the roof be clogged with something? And how easy is it to clean out the vent. The vent pipe was installed when the house was built over 50 years ago. It comes goes up between walls from the basement and comes out of the roof and sticks out about 3 ft from the roof. There is no type of cover on it that we can see and Chinese Elm tree hangs over that portion of the roof.