I replaced all the water lines in my 74 Sov center bath with Pex plumbing and crimp fittings. I did have the bedroom wall side of the bathroom removed, as well as the bathroom vanity. To get to the sink up front, I installed my fittings on the end of a ten foot stick of Pex, and snaked them over the wheel well behind the shower, and then cut off the bathroom end at the length I needed... It was all kind of loosey goosey, hit and miss, eyeball kind of work, but it came out very well. Without pulling the bathroom wall and vanity and toilet, it appears as if it would be very difficult. Sorry if this was no help... But it was how I did mine. Good luck with it!
Somebody ought to clean these windows. There is a tremendous buildup of gook all over them...