And if you turn on the furnace fan, same smell? Odd how the furnace outlet and black tank co-mingled... Is the furnace near either tank, maybe start there.
Is it possible you have a H2O leak and it is a mouldy smell?
Is it possible the previous owner had lost their cat and yer going to find it....
Are you able to get your hands on a bore scope? They are relatively inexpensive on Amazon.