You are bit limited up here, 2 sites spring to mind - one at Strathclyde Park, J5 M74 run by North Lanarkshire Council , and the other is a private site in Stepps just of the M80. About 30min & 20 min drive from Xscape respectivley on the motorways. Strathclyde Park is perfect if coming from the south, its just before the heavy traffic starts, 1 min of the M74 and has a Toby carvery literally across the road and a theme park 5 mins walk away with pub / snacks although it does get a bit roudy at the weekends with the chav / boy racers (the theme park, not the caravan park - its fairly well enclosed). Getting to Xscape is a doddle, M74, M73 then M8 - about 15 miles in total, if you are going to Xscape on the Saturday then traffic wont be an issue, it only clogs up Mon - Fri in rush hours.
Cannot think / would recommend any wild camping spots.
Google them and you should find it OK.
