There's nothing of any consequence under it.
You're good workmen but I say this for those who aren't.
Tile or linoleum Flooring to do the whole rv won't be enough money to worry about. So don't.
Climb under and take a look at the flat floor. Your soft spot appears to be right about at the frame rail.
Drill a little hole all the way through, from the top, at each corner you decide to template for the new piece.
Go under and see where they come out.
Set your saw and do the cut to the depth you determine..
It's really easy. You go in through the top while keeping an eye on the bottom.
I have my corner holes drilled and am about to whack a 19x36 hole in the rear most of the lower basement floor. You'll be able to reach through the rear hatch into the floor through a hatch in the floor.
I've done a floor repair on our prior toy hauler, same way, through the top after carefully checking the bottom. From the toy hauler I know what can't be reached from one side can be reached from the other. (I had a blow out come up through the floor and rip out the entire wheelwell and pull some stuff out of the trailer and throw it on the road.)
A harbor freight multipurpose(vibrating cutter) can be your true friend in all of this.
While you're down there, bolster the hell out of it. If a 1x1 was used but a 2x4 will fit....
It might be easiest to make a thin metal plate to cover the new hole from the bottom, screwing it in all around as you can reach and then set your new wood floor piece on top of your new metal plate, then screw your new wood plug to that plate.
You're now armored and it's a lot easier than cutting that plug to an exact exact size. Fill in gaps between old floor and new floor with gorilla glue mixed with the sawdust you made. Scrape the top flat, pick a nice new linoleum and make the rv yours.
Also, while you've got that flooring up, we're going to cut grooves in our floor and insert electric floor heat in the bathroom and between the beds.
(I'm telling you this electric floor stuff because you don't seem to have enough on your plate. I think it's only fair that your projects swell to fifteen projects... Like mine do.)