Why does my toilet bubble?

So my toilet in my master bath bubbles every time i shower, and every time my washing machine drains. The washing machine drain is on the same line as this particular bathroom. There's no problem with my tub or sink draining or the washing machine or any other drain in my house. After a shower or doing a load of laundry I cannot flush this particular toilet. On the occasion that I CAN flush it, if i flush it more than 3 times in a row, it stops up. When it does flush, once the water starts filling back up, it bubbles. We have flushed the vent in the roof and put a 25 ft snake down it. Nothing came up. We have snaked the washing machine line, and nothing. I can't get this particular auger through my toilet, so my next option tonight is to pull the toilet up.

Also note on this particular main line in order from farthest to closest to the septic system are: My toilet, my tub and bathroom sink, kitchen sink and dishwasher, and the washing machine. The ONLY thing not draining and the ONLY thing bubbling up is the toilet. Also, I have not been able to locate the clean out. I am assuming it's under the house. I live in a mobile home. I haven't been able to find it and i can't find it buried outside around the pipe that leads to the septic system.

I have also used a 64 oz container of Roebic Main Line Cleaner. After about 24 hours I heard a thundering sound from the bathroom. I was assuming some clog came loose? I flushed the toilet and it flushed faster than it ever has. After 3 times it wouldn't flush anymore.

UPDATE: I also want to add that there are TWO vents on the roof on that side of the house. One is directly over where the toilet is and it is covered with some sort of metal box. The other is open and is over the washing machine are. That is the pipe we flushed and snaked. Should I try getting the cover off of the other pipe and flushing it?

Any advice would be great!