Well, there is permitted camping in the valley between Wanlockhead and Mennock - a short drive from the M74. I would probably be a little wary about the boat at weekends, but other than Friday or Saturday nights in summer, I would expect it to be pretty quiet.
This is not a campsite, however, so you need to be self sufficient for hygiene.
There is a campsite between the M74 and Leadhills, I think, but I have only ever driven past, never stopped.
You could look on www.ukcampsite.co.uk for more suggestions.
