I have an older home, built in 1940. The original part of the house is pier and beam. The back addition is on a slab. I have a sewage smell that runs through the entire house. Currently, the back bathroom has been demolished, and just has a towel stuffed down the old shower drain. The kitchen has also been demolished, and does not have a working sink, just pipes ready to accept a sink. I cannot determine where the smell is coming from. I've sniffed pipes, looked under the house, etc. HELP.

Those demolished drains are the likely sources of your sewer gas, even though you've stuff with a towel. Re-install the traps and add water or seal permanently. While doing you remodeling be sure to verify all trapped drains have working vents to the roof.

Good solid advice by Balerherb,
If you can't reinstall the traps and fill them with water then cap off the pipes until you're ready to install fixtures. Sewer gas will leak past a rag stuffed in a open pipe. Good luck, Tom