taking bikes on a big trip

We want to take two bikes on our big trip and are still wondering how we should do that. Now we put them on a rack that is attached to our spare wheel (of our Jeep), but I am not sure if we could/should do that on a troopy? Would that put to much strain on the back door? Would we risk damage on the corrugated roads? Is a roof rack a better option? Or maybe converting one of those special fold-away systems for spare wheels? Surely I wouldnt want to put it on a tow bar as that prevents you from opening the back door..