I did a partial floor replacement on my last TT but the construction is the traditional type with wood over steel frame. So I can't give you advise on this repair. But I believe no matter what type of construction you are dealing with, you still have to remove the rotted floor to its closest supporting members so that you can secure your new floor structure to it.
One thing I do like to point out is that this type of floor damage is likely caused by water leakage from somewhere. It could be all the way up on the roof (in my case)or a nearby window. It may not through the door seal as it is too obvious. The source of water must be eliminated before the floor replacement.
