Here's what I've been up to. I removed the old battery door, and enlarged the opening to take an 11" x 24" baggage door. I built a wooden support frame inside, matching the trailer floor-to-wall curvature.
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I also added a wooden trim piece on the outside, again matching the trailer curvature.
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I attached both pieces with Powerfil 10 EH, a thick epoxy resin recommended by my fiberglass supplier. When that set, I applied fiberglass mat with the same resin.

At the same time, I repaired the cracked floor at the entry door, with a small wooden ski slope that will make sweeping out the trailer somewhat easier. Using the same resin, I fiberglassed ths as well.
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I then began work on the floor. My fiberglass mat came in a roll 38" wide. I borke the trailer floor into four main sections. 2 widths at the front for the beds, one between the fridge - stove, and the entry way piece.
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I carefully cut matching pieces for the individual floor sections.
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Now I was ready to fiberglass. I first rollered on a coat of resin in the first area. When it became tacky, I laid out the first piece on that resin. I then rollered another coat of resin over top, soaking the mat and bonding it to the layers below.
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It took about 1 litre of resin to coat the first layer on the wooden floor, about 38" x 75". It took about 2.5 litres of resin to soak and seal the mat. At the same time as I soaked the mat, I continued and coated the area for the second piece. I worked in batches of 1 to 1.5 litres because the resin thickened so quickly, even when only using 0.8% hardener by volume!

Over about 6 hours, I completed the entire floor.
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I mainly used a very short knap paint roller (1/8" or 3/16") recommended by my fiberglass supplier, with a 3" nattural bristle brush for the hard-to-reach places.

Next, is finishing the baggage door area, painting, and then installing a new baggage door.

Then the whole floor will be lightly sanded, so that I can install linoleum. I find that carpet traps odors and dirt, but mainly holds moisture, leading to rot at the cabinet bases. Linoleum provides for a quick &b easy clean-up. We do use a few area rugs from Ikea.

Victor

Nice job, Victor. Looks likes a lot of work, but well worth it.

Victor,
Thanks for the step-by-step description. I once made a copy of a '56 Chev pick-up rear window using the window itself as the "plug". I had to reduce the opening size to match the crawl-thru front window of our cab-over camper. The pick-up window was curved and actually larger than the windshield. I'd forgotten how I did it until your description of the same process!
Good job.
Kurt & Ann K.