Our camper did have a towbar receiver so we could have put a bike rack in there. Only issue was that we'd have to remove the lot to open the tailgate!
The bike carrier I have at the moment for the car is a cheapie similar to the one shown by lost, but it has two padded prongs sticking out which go under the bikes frame / top tube and you time them on. It's a pretty good system but I'd hate to trust it on serious corrugations over any longer period!
I guess like "not quite out there yet" said, it depends on what sort of bikes you have and how expensive they are as well. GripSport and other companies such as Thule make some pretty durable but also expensive bike racks, which you obviously wouldn't really consider if you've just got a couple of BigW bikes!
So it comes down to the sort of bikes you have, what sort of car and what sort of camper. You would also think about weather you're likely to forget the bikes are on the roof when driving under something low (carpark in the city or tree branches off road). Do you have to take the bikes off the camper if they are on top of it every night and do you mind doing that? Is there a spot you would perefer to keep the bikes where you wouldn't have to remove them unless you want to? If so then you've got to figure out how you can mount them there!