Welcome aboard Nadya. I think it's unusual that you dont find a seventies vintage trailer with some floor rot. I replaced the rear 4 feet on my trailer about two years ago. I also wanted to minimize the amount of rearranging I did, but in hindsight, wouldnt have minded making a rear bath into a rear bedroom and then moving the bath forward. If you want to see what is involved with a rear bath rennovation, then look at my blong <points down>.
It is always a good idea to assess the condition of the frame before you get too far into the project. To do that, you need to pull off the bellypan and remove the pink fiberglass insulation. Afterward, you can use some of the rigid closed cell foam boards that won't hold moisture against the steel. I would also focus and making sure you dont have any other leaks that could cause any new work to go south in a short time. Leaks are sometimes hard to find on these trailers since the water can travel a bit between the inner and outer skins. But they do leak and you do want to control that as much as you can. Good luck and post some pics of your new rig.