Our toilet downstairs plugged up. Had lines cleaned out vents checked, sewer lines changed out to the tune of $4000. Installed two new low flush toilets. Weren't we surprised after cleaning plug ups every day for two months, to find that all the expense and aggravation were for naught. We still have the same problem. We've had the plumbers back in yet again to use the camera. The only thing they found was a "belly" right underneath our laundry room floor. The pipe sits partially filled with water. We've now had the downstairs toilet refitted with high flow innards to push more water. However we're still plugging up every day and air comes up fro the toilet bowl. Any ideas. We're desperate.

we're still plugging up every day and air comes up fro the toilet bowl You plumbers should have picked up on that.
Bubbles indicate a blockage, A bell in the line wouldn't cause that to happen.
Let me explain,
You flush and the discharge goes down the drain line and hits a blockage and bounces back sending a bubble of sewer gas ahead of it. There's your air. What you have is a blockage they haven't picked up on yet. Did they SewerCam the entire system? No broken pipes? Well something's causing this and your plumbers just haven't found it yet. Keep looking! Good luck, Tom