You are very welcome, Magsy. I've spent a fair bit of time researching these Campbells as my gt gt gt grandfather (Graham Campbell) was the steward at Kilmory for Peter Campbell III, and is mentioned in his will. Graham worked there from at least 1805 to the 1820's, so I've been trying to find out more about him.
I have Marion Campbell of Kilberry's book "Letters by the Packet" which has some correspondence between the Campbells in Jamaica, Kilberry, Kilmory etc. (they were all related). She died in 2000 but had transferred all the papers from Kilberry to Argyll and Bute Archives. They are called the "Kilberry Papers", and I believe include the extensive Campbell trees.
In a letter from Marion's book dated May 2nd 1732, from Peter 1st of Fish River, Jamaica, (also Kilmory) to Archibald Campbell of Knockbuy, Peter writes about their youngest sister Molly being "most in want" and "very base of apparel". In a footnote Marion Campbell writes:
Molly (Mary) was probably unmarried and living with her father. Eventually she married George Scott, Controller of Customs at Greenock - a useful contact for her brothers when they shipped their produce to the Clyde.
When I saw your post Magsy, I immediately thought May must have been in one of those advantageous Campbell marriages, that's why I think it a strong possibility that she's related to these Jamaica Campbells. I know from my gt gt gt grandfather's travels with Peter III that they spent a fair amount of time in Perthshire, too, so they probably had property there as well.
Definitely worth looking into, I hope your husband thinks its worth a trip! I'd love to go to Lochgilphead myself but I'm stuck in freezing cold Australia (minus 8 this morning!)
All the best with your fascinating search, Billiegirl