Sewage smell.. propane leak?

We still can't figure it out, but will report back when we do. The black water tank doesn't smell anything like this smell, and the grey water tank has no odor. Because of this, we have determined a small animal is either living or has died in part of our trailer. We checked the space under the shower as thoroughly as we can, but there are no droppings or signs of anything. It's a mystery for the time being.