I'm in the same boat as Durans there. I've been reading up on it and can't quite get the light of day on it. I have a 75 31' Sovereign
and I have looked a different threads dealing with frame channel depth. Mine has the 5 inch channel frame. I did the rear end sag test and i couldn't notice much movement difference. The exposed part of the frame at the very back does have some cancerous rust, but the trailer was in florida for the better part of 25 years. I have not been able to find any water damage yet in my trailer. I do plan on keeping it awhile, and one thing I am going to have to do is to re enforce the frames that hold the blackwater and aux tanks. I have not yet dropped the belly skin in that area, which I intend to do because there is some extensive rust on the exposed part of the holding frame.
After reading that basically all 70s' model 31' trailers will be plagued by the rear end sag, I have tried to find links or pic of what the frame reenforcement kit looks like. I have the service manual for my trailer and have looked at the breakdown of the frame and have come up with a couple of ideas on how I would do the beefing up, but I was curious of what was already out there.
On the end note I am totally happy with "Silver Team #2", it will take a little bit of work to get her up to par but in the end it will be totally worth it.