My 40E does the same thing. If the sun is on the otherside of the rig you will notice the shower stall will separate from the wall also. When I first saw it on mine I was not sure what the cause was, but I ran into another 40E owner and his also does it. If you look, it is not just the vanity, but the wall behind it, the washer/dryer cabinet. Not sure you can do anything about it.
2007 Fleetwood Excursion 40E
2007 Chev Classic CC LB D/A - Toad
2013 Jeep Sahara Unlimited - Backup Toad