Repairing rot hideout 178LHS

[Roger356CCB]

2017 sill plate under floor and under outside walls badly rotted from water intrusion at underbelly membrane tears where the bolts pass thru and attach to the frame.
Some subfloor will also be replaced.
What type wood for these repairs?
I was thinking Douglas fir for the sill plate and regular 5/8 oxboard subfloor.
What product to patch underbelly?

 

[Holdmybeer]

It takes a lot of water over time to rot out wood. Difficult to imagine how a tear in the underbelly would precipitate that. I'd look for a leak coming from somewhere above.
Otherwise, if it's a soft underbelly, there a product called " Bottom Board".which comes in rolls ( 3' or 4') and is attached by spraying both surfaces with an adhesive. Any Mobile Home Supply house will stock it.