Had a similar problem last year with mine, but I couldn't disengage the clutch properly. The problem is corrosion and a build up of 'clutch dust' on the gearbox shaft / driven plate splines, campers suffer because the tend not to be used over the winter. Only option is to have a new clutch fitted. There is no way of cleaning it up without splitting the gearbox / engine and if you've got that far you may as well put a new clutch in. Cost £400 but made a world of difference as it had obviously been 'dragging' for a while.