If a combined drainage outlet it could be a fat plug from the kitchen sink if you've poured fat down plug hole.
If basin and shower in bathroom has it's own drain outlet then most likely to be a hair ball. We all loose a bit of hair bathing or showering which heads down the drain forming a fur ball like a cats one.
Have a look under the campervan see if the tail end of shower drain comes through floor and the drain pipe is connected if so disconnect pipe and put a drain worm down the pipe [flexi drain wire with brush on the end. You may have to take the whole pipe off if it's a hair ball blockage, which most shower drains are. Yes we all loose a bit when showering and bathing. Failing removing at shower end you could cut the pipe and use a pipe joiner check you can get them local first at your nearest camping and caravan outlet tell them what you want if not sure
Bob
June 2010