Hi Sheila, I was gone a few days without WiFi and missed your post, but mystery smells seem to be an RVers fact of life. There are so many possibilities and as Joella said, you'll probably need to have it checked out in Q. I know we once had a smelly problem that turned out to be a window partially open at the other end of the rig which created a draft toward the living area and pulled in the holding tank's roof vent smells. And, yes, sink drains can often smell like black tank smells.
In the meantime, how about trying all the different combinations of air flow you can think of. Put a vent on exhaust to pull the smell away from you. Be sure to put water in each and every drain, including shower, laundry, and outside shower hookup drain. And, by all means, have that grandson check out everything. Maybe your fix is simple. Then again, you may have to pay a pro. Good luck, my friend.
p.s. Perhaps Larry discovered a water leak and cut the hole to let out the water. Could have been from a holding tank or whatever.