Plumbing – Laundry Box Overflow/Gurgling

Without more detail (type of system: septic and leech field or city line) it's difficult to answer with accuracy. Here are some general ideas that could help clear the lines: stop using drain clearing solutions (caustic or acidic). Used excessively they can corrode cast pipe and ruin the balance of septic tanks. You'll have better results clearing a stopped drain with a plunger (faster and safer) or a 3/8" cable. I'd look for the cleanout for the main house line, usually a few feet off the outside wall. It could be in the cellar (if you have one), but try snaking the cable from there. Lastly, run a garden hose that has been pushed down the roof stack to clear any blockage that may be affecting the venting in the drain line. Also, if you have one , make sure the septic tank doesn't need to be pumped (this probably isn't causing the problem).

If there is not any backup evident at the outdoor clean-outs or tub drain when sinks and toilets are flushed together, then I would say the septic is not slow draining and air bubbling into the toilet requires that the soil vent serving the toilet be plugged some other way. Since the toilet vent line was recently cleaned and found to be plugged, I would surmise that the vent has somehow become plugged again.

Offhand I can think of two possibilities:

(1) The toilet soil pipe has a sag near the connection to the soil vent such that the branch to the vent tends to collect sewage and become plugged;

(2) The toilet soil vent is connected to the drain line incorrectly (incorrect wye, installed backward, etc);

There are companies for hire that can run a camera through the lines to find the problem if you are so inclined.