Ah! Hold your horses here! I came across this term quite recently in respect of a family member. 'Scavenger' can have several meanings, as I subsequently discovered. It all depends upon context. It was a term used 'locally' as in someone employed by the local Parish, or similar. I came across two examples, from different areas, as Scavenger-Constable'. One referred to an appointee responsible for the apprehension and arrest of the 'non-contributing' fathers of children born to un-married mothers. This particular job required the writing of reports for presentation to the Magisterial authorities; therefore a job for a literate and educated man. The other example referred to the policing of church-yards (combating resurrectionists/grave-robbers perhaps?).
So, 'scavenger' seems to mean 'a finder' of things, be they people or whatever, for some later purpose. Perhaps a 'scavenger' of rubbish was someone who 'sorted' through waste, to extract stuff of potential value, rather than someone who just collected, and dumped, the waste? A bit like the ‘Toshers’ who worked the sewers for lost valuables.
However, I have never come across the word in connection to 'night-soil' collectors, or those who collected urine (‘Piss Willies’) for the wool/serge 'fulling' trade ... although these too, collected for some eventual 'other' use/product/purpose etc. Perhaps this term became debased, and narrowed, to lesser duties, as we gradually became a more 'modern' society ? As has happened to the term 'Night-Watchman' etc.