Thanks for the info. The pictures were very helpful.
I finally got a chance to take a look at it, and there was no way to remove the shelf brace under the broken area. I had to take the whole top off. Once it was off, I saw how bad the wood was rotted, and decided that I needed to do a whole trunk rebuild. Finally finished that and am now repairing the broken area of the trunk top. There were tons of small cracks in it. I used a thin mixture of ABS/MEK to flow some plastic from the top into the cracks, the did the same from the bottom. Then I used the ABS/MEK mixture to "glue" some fiberglass cloth to the underside, and built up the area with several coats of ABS/MEK. It's ready to paint the patch on the top, and install it. I found a couple cracks in the lid that I will fix before I put the lid seal back on. Hopefully I will be done by the weekend.
Of course spending so much time with my camper, I noticed that the roof is starting to get small cracks around the edges. Guess I need to start saving for the shipping costs of a new roof since it I'm the original owner. What next

I think my new trunk looks better than the original. Better wood, and lighter too

I made the shelf removable so I can get to the underside without having to remove the trunk top again. Hopefully I won't have to do any more repairs, but if I do it will be easier. I also added an outlet by the water heater so I can plug in my electric element when I need it.