Definitely use fiberglass strips on the inside. Many people have also chose to use a fiberglass strip on the outside as well.
Fiberglass cloth does come in ribbon like rolls of varying widths. Those rolls are called "fiberglass tape" and they are much easier to work with than hand cutting strips from a large sheet of fiberglass cloth. Once the resin has cured you can sand the slightly raised woven edge of the tape to blend it seamlessly into the surface.
As to plugging the holes.... as are doing a fresh layer of materials it is best to use like materials with like materials. In other words use a compatible hole filler that goes with the resin type that you will be securing the fiberglass tape with.
As I personally like to use epoxy resin I fill small screw holes with Epoxy Putty Sticks, the kind you knead to mix the inside layer with the outside layer of material.
If you are using polyester fiberglass resins then you can use Bondo types of filler that work well with the polyester resins which you mix up and trowel into the holes to level the surface.
I choose epoxy resin because it has more strength and you can get versions of it with longer open time. However it is also the more expensive type of resin.
You can make the job of applying the tape easier faster as well as a whole lot less messy. I learned this method of application from a professional marine fiberglass repair shop. Do as follows:
Cover a table top surface with a sheet of plastic. Then roll out a length of the fiberglass tape on that surface. Pour some of the resin along that length of tape. Using the small flexible squeegees sold
for fiberglass work push that resin into the tape. Use just enough resin so that see the fiberglass tape turn from white to clear. You will quickly figure out if you put on too much if it drips resin all over the place when you lift up the tape. Then with a partner lift the ends of the tape up apply it over the seam line using the squeegee smooth it into place. After it sets you can squeegee more resin onto the tape surface if needed.
If you are really worried about the strength of the join you can put a second layer of fiberglass tape on the interior of the trailer only. To do that purchase the fiberglass tape rolls in two different widths. Put the narrow tape on first then apply the wider tape on as a second layer of fiberglass cloth. Having that staggered width of wider over narrower makes for a much smoother surface finish rather than stacking two layers of the same width on top of each other.
It is not all that bad of a job to do with this method. Do work with a partner if you can as it will make it go much easier for handling the installation of the fiberglass tape. That tape cost more than sheets of cloth but it is so very much worth the extra expense versus dealing with fraying edges of glass fibers which come apart from the weave while spreading out the resin. Quick, neat and easy versus a messy struggle.