At the 1881 Census, your folks were at Campbelton Rd 'Vich ...dhaorin' Knapdale South. I think this should be Aorinvichricatroth, South Knapdale which I think must be near Inverneil on Lochfyne just south of Lochgilphead. To my knowledge by the way, there is no specific place called South Knapdale: it is an area about twenty miles by eight or nine, so it is difficult to know whether the above address is the one you quote.
The family at that time was shown as Archd. Ferguson 52 fisherman; wife Isabell 49; Archd. son 24 fisherman; Colin 22 fisherman; Margaret 20 Gen servt; Angus 17 fisherman; Duncan 13 scolar [i.e. still at school; Catharine 7 scholar; and John 5 scholar.
All were born in S Knapdale except the father, who was from Kilbrandon [which is the area to the north where the slate comes from, including Easdale and Luing]. There seems to be just one family of Fergusons originating in that area and all the men seem to be called Archibald. equally interesting, the family's next door neighbours at the time of the Census were Donald ferguson etc. He was also a fisherman born in Kilbrandon and eioght years older in the Census [few ages are exactly right] so I would put money on his being your Archd.'s brother.
All the Beith's in Argyll at the time were from Campbeltown. Keller's in turn were effectively from Kilcalmonnell.
Hope this is of interest and gives you a start: you should be able now to go back to the Scots Origins database and find Archd.'s birth/christening etc. Campbell may be more difficult because of numbers though you now know Isabella was born in Knapdale south. Good hunting.