So I've concluded that my rear end separation wasn't leaky plumbing but leaky tail lights. If you have suspected any of this it could be a fairly easy fix and I'm sure this has been posted before. So the taillight cups come out (from the outside thank goodness) and you can probably put some vulkem around the cups but that would solve only half the problem. Also pinpoint around the beltline rub rail (top one) where water can gather and siphon on top of the taillight. I have some pretty big puddles sitting in my C Channel. Cool thing is you can attack most of this stuff from the outside of the trailer. I have a 77 excella and these taillights stick out quite a bit but I'm sure they happen on other ones as well. Note that this leak deposits directly at the main frame rail bolt/c channel connection so the rust just killed that whole critical point of connection and helped accelerate the separation as well as axles I'm sure. I'm not the original owner but after taking out the floor I could see that a lot of the rot only happened at the corners and the steel angle stock at the very rear of the trailer. Hope this helps but I'm sure I've seen posts on this before.
Water stains where it enters taillight cup. It also enters around the taillight housing at the top
You can probably seal around here with vulkem or similar sealant
Water pools here and enters most like the main source of water entering the rear
Pooled inside the C channel with nowhere to go but rust out the bolts and leak though to the subfloor