I haven't used Nature's Miracle, but if you can't find it, I have one from a commercial janitorial supply called Bio-Zyme. It kills the smell of cat pee after a couple of applications. But I'd get rid of the rug anyway if the trailer is that old.
No doubt the gaskets in the sewage holding tank need replacing. It's not that hard to do, but it's a job no one looks forward to, and you need to be prepared to take a really good shower immediately afterward. You might consider changing gaskets before jumping through all of the other renovation hoops, to rule out a real obvious source of foul air first.