I remodeled my house in 1994. I had all my plumbing replaced. Starting this year, I noticed a sewer gas smell when the air was on. I'm a carpenter, so I asked a plumber on the job site about this. He told me to check for any dry traps. I have a floor drain in my laundry room, so I poured water in it. It still smells.So next I go into the crawlspace, because all my ductwork is down there along with my plumbing drains. I figure there might be a loose pvc pipe somewhere. Everything looked OK. I was just about to crawl out when I noticed my cast Iron sewer pipe was not sloped at all. I put a 2ft level on it. It was level for about the first 10 ft.

I think it should be 1/4 inch per foot slope? Would this be my problem? My toilets flush, my sinks drain.
I would appreciate any advice.