C1 is for vehicles over 3,500Kgs. Almost all campervans are 3,500kgs and under, and so can be driven on B category license.
Mercedes Sprinter, Ford Transit, etc are almost always 3,500kgs.
Some are 4,600 and some 5,000, but these are normally clearly marked as such, and almost always have double rear tyres.
You can assume 3,500 kgs unless you read otherwise in the documentation. The V5 log book will list the weight.