From what I can see, and what I know of how a similar trailer is put together (U-haul), that looks like a relatively straightforward repair.
was made with polyester resin. As you probably remember from your past fiberglassing, there are two types of bonds: Chemical and secondary (or physical).
You can use polyester resin, but it does not have very good secondary bonding characteristics, as compared to epoxy. Now, it would probably be fine for the repair you have there; but, on the other hand, why not use epoxy? I actually prefer it because it does not have the (to me) noxious smell that poly- and vinylester have. It is slightly more expensive, but in the amounts you will be using it will be negligable. And the secondary bonding with epoxy is superior. (You can no longer get a chemical bond because that has to be made within hours of the original fiberglass being laid.)
I would use a biaxial reinforcement (that means the fibers run on the diagonal and they all provide strength; whereas a 90ļ type cloth would have half the fibers running parallel to the crack and not really helping). A "tape" style will make for nice, neat edges, but you could cut repair strips out of a larger piece, too.
"biaxmat" is a reinforcement that has a layer of biaxial cloth stitched to a layer of mat, and it would work well. Now, like with the resin, you could get away with something else, as it's not a major repair in terms of strength.
So I would proceed roughly as follows:
1) De-wax with something like Interlux 202 Solvent wash
2) Tape/plastic off surrounding areas
3) Sand area to be repaired (for smoothness and tooth).
4) See if it smoothish or if there is a "rut" that would be hard to fiberglass (if so fill it first with thickened epoxy).
5) Cut a couple of layers of reinforcement (cloth) to fit; stagger the size slightly (you may only need one layer, depending on weight
6) Wipe surface with acetone or denatured alcohol
7) Wet out surface with "neat" epoxy
8) Wet out reinforcements with neat epoxy (to the side, on plastic or in a dishpan, etc.)
9) Apply reinforcments to prepared area
10) Brush/squeegee to remove air bubbles and excess resin
11) Leave to cure (tape cloth in place if it wants to fall
Since that area (I think) gets covered by either carpet or a bench or something you only need to make sure it is relatively smooth, so cosmetically it is not too demanding.
Schools of thought vary on whether to put the larger or smaller piece on first (you probably don't even need more than one layer, really), but either would work fine here since you are not going to be sanding/fairing.
This will now be the strongest area on the trailer
I would just aim for covering out to the white section, but not overlap it.
You probably remember this, but it's important to wear protective clothing, gloves, eye protection, and a respirator.