We have a single story ranch style home. There is a small crawl space underneath, but it is VERY tight - We seem to be having trouble with odor coming from the farthest bathroom towards the back of the house. When you go outside, the vent underneath the house which happens to be right below the bathroom, STINKS like poo. Some days it is worse than others. Every once in a while, we can smell the smell inside the bathrooms, but it is the worst coming from the outside grate vents. My husband took a flashlight and peeked through the vents but could not see any moisture leaking anywhere. Could it be the ceiling vent that the gasses are supposed to go through that is cracked/clogged? I have read about that in a few of these blogs, but do not have a clear idea how that works. Any ideas on what to check for? Thanks everyone.
Also, we can see the vent on the roof - but - how do you check it for a clog?? Maybe a silly question, but we are very inexperienced!

You going to have to go in and look first hand. Look for breaks in the piping, water on ground, piping open, piping not completed. Look also at underneath of floor to see if water is leaking from above and you have a mold issue. If you have black mold you will be desperately wishing you wore a mask to prevent breathing in mold spores.