I keep a clean old large bath towel in my truck at all times. Multiplicity of uses: cleaning up, sling, padding a splint, pillow, beach blanket, covering valuable objects to keep them out of view, easily ripped up if need be for wound dressing, headband, shade, etc.
A few "clean" shop rags, an old bath towel, and a nice queen sized heavyweight wool blanket (along with 1/2 case of water, some MREs, coffee powder, all the fixins for coffee/tea, and an old Coleman Peak 1 camping stove (and don't forget the e-tool and a roll of TP - it's amazing what you can fit in a 5 gallon bucket or 2