What is the indicator of a rotted out roof. I walk on my 2005 FS2600 roof and it gives a little under my 200 pounds but I can also feel the trailer rocking on the suspension. In the front corners of the roof where the TPO meets the aluminum siding I rap on the roof with my knuckles and it sounds solid and it is not spongy; I had evidence of water seepage there because the screws were rusty and WW did a poor job of sealing the area.
I too have a slight sag in my roof by the AC unit, rear corner of it touches the roof. I'll have to check to see if I have finger joint limber on my 2005 which was built in 2004. No leaks around the AC.