Nick, You can see my floorplan at http://www.airstreamphotos.com/photo...500/ppuser/139
There are only two pipes which run under the floor. They run from under the shower across to, I believe, under the kitchen counter. If not, just aft. As it would be necessary to drop the fresh water pan, if not the tank itself to replace them, I just left the tubing in place. The polybutylene tubing is perfectly good stuff, it is the connectors that fail. I picked up with Flair-it fittings and Pex at either end.
Replacing the plumbing around the low point drains was a pain - litterally. There is not much room there.