[Mike and Jan]

Start making jokes now, well before the trip. Like saying you sleep in the nude and do you need to bring a nightie if you're sharing a room! That'll get their attention! Or, mentioning that sharing a room isn't going to mean you're not going to enjoy yourself in bed.
If they don't jump in then you might just let it drop. But, I'll bet they'll spar right back and by the time you're there the mood will be right.