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.

While your drainage needs slope I don't think it's the cause of your smell. Smells are the hardest to track down when you're not there. Give all drains the "sniff test" and attempt to locate the source. Good luck, Tom