We are having an issue in our house where the shower backs up but only after you run it for a while (5+ minutes). When the water backs up and starts to drain, the toilet water simultaneously sucks in like a vacuum. It seems as though there is negative pressure sucking it in and bubbling for air. Plumber #1: Said it is a main line clog. But he said he can't find the main line cleanout. He did not want to check the crawlspace. They said the only way to do it would be to remove the toilet and snake through there for $500. He also said there is no cleanout and we could install one for $750. Plumber #2: Could not find a cleanout (didn't want to go into crawlspace). Tried to snake through a vent pipe on the roof but he said he couldn't really make it work since all of our vents are 2 inches. He did not want to go through the toilet because he said it is messy and might not work. Plumber #3: Said we have to use a camera to find the obstruction then install a new cleanout and snake for something like $300-1000. I'm lost. I'm not a plumber, but I have done some minor work and snaked pipes with my dad. Something about this whole situation doesn't seem to add up to me. What makes matters even worse is that the backup problem seems to have gone away. Though looking under the house it looks like there is some sort of leak/overflow of liquid from the pipe running vertically from the house to the horizontal line it connects to (I'm guessing this is the main sewer line?) I'm going to try going under the house today to see what I find. Just thought I'd ask here for some expert's opinions.

I just bought a house and have been noticing a sulfur type smell when I run my wash. I noticed the washing machine drain is directly hooked to the sewer pipe not allowing any proper way to vent. Could this be sewer gas I'm smelling? & what's the fix?