Well, not to rain on a parade here, but we too had discoloration in our vinyl flooring in our Arctic Fox, and sadly it turned out the door sill had been leaking for 5 years and that was the very sign that the plywood was rotting away under the vinyl.
Happily for us Northwoods MFG took our RV back even after 5 years and rebuilt the affected areas and installed new flooring throughout, and not a penny out of pocket for me except to deliver and pick up the unit myself from the factory in Oregon.
We live in the rainy NW, but do keep our RV's inside, and I see you live in the desert, so I don't know how much of a threat dry rot would be. Best of luck, Hans