In the one foto of the bottom, notice how one, maybe two of the boards are a little shorter than the rest.
Cut all of the boards full length, and fit in as many as you can by installing then at an angle, and then pushing them straight. A small cut-off at two opposite corners will help with this. (see sketch)
When you get down to the last couple, trim them a little, and use a couple of big pry-bars to flex the board until you can get it past the steel. If you can't make it happen, trim a bit more.
Even if you have to trim off a full 3/4 - 1 inch, there will still be enough to hide the ends under the metal lip. Put countersunk screws thru the board and steel crossmember to hold it in place if you feel the need, altho once covered with the rubber mat, you'll never notice the small gap. If you do put in screws, use stainless steel or brass hardware so you can get them loose easily if the need arises.
The boards in this trailer appear to be rough-sawn Oak, and where you live, you shouldn't have any trouble finding more material. You may find that you need to have one board narrower than the rest to get a good fit. Unless you have a full sized table saw at home, have the lumberyard do this for you. My trailer has between 1/8 and 1/4 inch between the boards; this one looks more like 1/2 inch, and that's probably just fine, but I wouldn't go much wider than that.
(edit to add) The soft spot doesn't really look too bad. As a temporary fix, install a square of scrap Aluminum sheet over the damaged area, and extending onto the adjacent board; something about the thickness of a street-sign would be ideal. Put two small screws down into the wood to hold it there, and bury it under the rubber mat. Periodically inspect from below, but I wouldn't worry too much until you notice the rotten wood creeping down toward the bottom of the board(s). Then do them all.
(edit again) Fill both sides first, and save the final board for in the middle. You will be able to get the steel angle-iron to flex a little bit in the middle, but not at the end where it is welded.
Steve Jernigan KG0MB
University of Colorado
Last edited by george the mule; 04-13-2019 at 06:37 PM.