Done this a few times and it is indeed difficult, especially in a corner or in a bathroom. Why the RV guys don't seal the edges of the flooring even where they cut for a waste pipe, is difficult for me to understand, that said, if you do the work a few tips.
Prep - remove everything that can get in your way, EVERYTHING. Toilet, Cabinets, get it all out of the way. My first repair I didn't do this and regretted it big time. You'll find that the repair area is bigger than you think, like you might decide to extend the cut-out until you get to a underfloor brace.
As mentioned seal the edges of all wood going back in. Aluminum roofing paint works well and lasts longer than the c--- the RV guys put in.
Remember, most RV's start with a floor completely covered with vinyl flooring called "sheet goods". Carpet, Pergo flooring or whatever is installed over this. Sorry, but it has to go too.
Be sure to seal any place where the floor repair goes to the outside, like under the door you mentioned.
If the bad wood goes under the outside wall, it still must be replaced, but I can tell ya, it adds a lot of time and effort to the repair.