Drainage problems

Has anyone found that the drainage in a Jayco heritage with combiined toilet /shower and hand basin has a very slow drainage from the hand basin and actually stops draining? Even after using the old 'draino' and modifying the plug area to take a flexi drive there is only a 3/4 meter access. After dropping the drain hose underneath and accessing the drainpipe the flexidrive can only penetrate 3/4 meter, does the system have a 't;'joiner to take the overflow from when the hand basin is raised? If so is the only access via taking the whole handbasin toilet system out to get into the drain area behind the handbasin? Any help gratefully appreciated

Geoff Smith, QLD