I have a Monaco that has the same issue. I’m also hoping to find the answer. Mine comes the strongest from the closet behind the head and over the hot water heater. Never had a cat but smells like you say, 100 cats sprayed in there.
This is a 2 year old post and the OP hasn’t been back since. That bugs me when you try to help someone and they don’t have the decency to get back to us with the results.