When my daughter and I did a tour last summer, she introduced me to 'cleansing cloths'. These are dry microfiber cloths that have soap on them. When you wet them, they get a good lather going and work well to remove grease, grim and road gunk. The best thing about them is that they are dry and are easily stored in a plastic bag. They are also way better than bar soap because bar soap invariably gets all soft and messy after use and bouncing around in a bag all day. It usually took about 3 of them per shower for me. There are several brands (your wife will know which ones are best ) but here is one example. You'll want to make sure whichever brand you choose it's the 'dry' cleansing cloths, not the moist towelettes.

By the way, I use the Packtowel. It comes in many sizes and is easily laundered. I have a very large bath towel size and a smaller hand towel size which I use for drying dishes.