Finally! My folks came down to visit and my dad, brother, and myself replaced the old plumbing and installed the new subfloor! We ended up putting the new subfloor on top of the old one, patching the rotten pieces first. This way I have more insulation and a solid subfloor without having to remove the old bolts.
I am moving the bathroom forward from the back of the trailer so we installed the drain for the shower pan and also moved the sink back so that the drain for it can be hidden inside a small wall with the vent which will also support the counter.
The drain vent for the shower is being carefully hidden under the shower pan and the pan had to be moved a few inches closer to the center of the trailer due to a steel frame bit being in the way of the intended location. All in all, we did some good work and now it is on to installing the water and propane lines!
I also started to sand the ceiling, getting some of the bad paint lines, and am also going to be cutting a piece of the old shower stall to fill in the right rear corner of the trailer interior that does not have a wall panel.