Not sure about the specifics of the Roo, but I fixed a small soft spot in the floor of my Flagstaff Ultra Lite and it's not as clear cut as one might think.
I'm sure the ROO floors are the same construction, it's 1/8Luan with foam in the middle of it like a sandwich.
I had a small soft spot 12"x6". However, when I pulled the vinyl floor up, I discovered it was more like 3'x3'. I ended up ripping out all the bad wood that was rotted all the way to good wood. I took special screws from Home Depot which go through Wood and Metal. I then took pressure treated 2x4's and ripped them down to size to make them even with the current floor. I screwed the pressure treated wood to the aluminum framing and essentially made my own subfloor. I filled all the voids with insulating foam sheets that I cut down and then I put Luan overtop to make it match up with exisiting good floor. I ended up ripping the vinyl up and put down a new sheet of vinyl that was hard wood floor look alike. You couldn't even tell the difference after it was finished but, it wasn't a quick job and I'm pretty handy.
If you are very handy, you'll probably find it to be not horrible of a job however, you will probably be amazed how your small soft spot isn't likely as small as you think.
2012 Keystone Sprinter
Still sticking around for the good people and advice.