For those recommending replacing or removing the cracked part, isn't the white part of that rear cap one piece? That would be a pretty awkward job to remove for the average person. Certainly if working alone. It looks like the lower blue section is a separate piece, but can't tell for sure. Seeing as the crack stopped at the white/blue interface, I figured it was a separate piece.
On our rig, there is about a 4"- 6" gap between the inside of the rear fiberglas cap and the structure of our interior bedroom wall. It's wider at the bottom and narrows near the top. If this rig is built the same way, there isn't any chance of interior water damage if water gets into the crack. Also, if there is that much space, I think Veebyes recommendation to lay some supporting fiberglas on the inside is a great idea. It doesn't have to look pretty as no one will see it, so you just have to be able to reach up that high and smooth out the fiberglass. (Wear old clothes, long sleeves, goggles and gloves! LOL) If you can get things mostly aligned and solid from the inside of the cap, that should minimize what you have to do on the outside because the strength will be provided from the inside repair. The outside work will mostly be to make it look pretty. Just make sure to thoroughly clean that inside area before applying the fiberglas. It's likely to have a lot of dirt and road grit & grime collected from over the years. Clean and scrub it, if possible.
Veebyes also mentioned preparation. THAT is the key word for any type of body work. Make sure everything is properly cleaned and materials are applied according to directions. Grinding and then sanding with the proper progression of sandpaper grits. As someone else mentioned, if it doesn't quite turn out right, grind out your mistake(s) and try again. But before you spray the paint, it needs to be "perfect" or as close as you're willing to accept to "perfect". The paint WILL SHOW any imperfections in the process. Then again, it's an older rig, so I know I could easily convince myself not to dwell on "perfect". The likely mismatch in color and gloss will tip off anyone to the repaired area. But the only way around that is to paint the entire rear cap. As I mentioned before, if it's mostly unnoticeable unless someone takes a good, second look...I'd be happy.
Heck, depending upon how much longer I intended to have the rig, I could probably even convince myself to make it water-tight (a good, white tape, perhaps even Eternabond?) and just live with it.
I hope to hear what your findings are, and like others, I'd like to hear how this happened. Many of us flat-tow, so whether it was a component failure or operator error, understanding what happened in your unfortunately situation could possibly keep us from the same mishap.
Good Luck,
~Rick