If the fiberglass molded "furniture" is sound, meaning not broken, You might want to keep it. The concern I would have with replacing the interior fiberglass with wood is the added weight
. These little trailers were designed to be lightweight and the trailer is designed accordingly. If you add too much wood, the added weight
could cause a weak spot in the frame to break too. Ian has a lot of good information on Bolers on his website that might help you.
Another very important fact when removing the interior components of these trailers, the closet and the kitchen cabinets support the roof. It looks like yours still have the supports on the left side of the kitchen counter by the sink. Those twisted iron bars are functional and provide support for the upper cabinets and the roof. If you remove the kitchen cabinets, especially the lower ones, you will need to support the roof or it will sag and be likely to crack or break. I hope this helps.
There is also a very informative thread on fiberglass repair. Search for fiberglass in the forums and you'll find it. Good luck and I look forward to more pictures on your progress.
Sorry, I missed that you already removed the lower kitchen cabinet. You might want to prop the upper cabinets or roof with a piece of lumber that's padded on the end contacting the roof. The sag may not be noticeable, but over time your roof could be damaged.