Well what'da know? I actually worked on the camper this evening! It felt good to get back in the loft, despite it being to do body work.
I hadn't been looking forward to dealing with those big sags that I got when trying to apply thickened epoxy filler to the rear portion of the street side wall fiber glass weave. I thought the thickness would have been good enough for the amount of tilt I put on the cabin (versus trying to do a pure vertical application) but, as you may recall, it worked against me.
The best method of dealing with the sags turned out to be using a small hand plane (one that I am less fond of) to shave most of the highs down. Then a lot of long board. Some medium board for the detail work around the marker and porch light wires, and along the edges. Then the palm sander with felt pad to scuff up all of the low areas so that the next round of filler has some tooth to grab onto.
It was a lot of sanding and it's not close to 'good enough' yet. Figuring that the rear part of each wall is about 2/3 to 3/4 of each wall, and that the front wall and hatch are about the same as a wall; if they each take 2 to 3 rounds of filler and sanding, then I only have to do this 20 to 30 more times before being ready for primer. Is that sarcasm? Probably not.
So with this in mind and not trusting that I won't get more sags, it starts to look more and more like it would be worth the effort to flip the cabin on its side to get a flat surface to work on.