I’d opt for the fillet. You will never see it again once you’ve completed the camper. Don’t do the strap thing. Once you cut out your galley hatch, it might change its shape unfavourably because of the stored tension you applied earlier. The mold should give the shell its shape, not a ‘secondary’ element inside.
I think this particular (temporary) gap is a fairly common occurrence – it happened to me and I saw a few other posts about it. I figured the best way to deal with it was to try to keep the camper as close to spec as possible; I wanted to close the gap to keep the pieces together as designed. The disadvantages of having a small work area turned into an advantage because I was able to use some scrap lumber wedged against the wall of my shop to hold the sides flat against the galley flat while the fillet and taping job cured. See the attached photo. Like jmb builds, I would worry about the potential distortion of using a strap to squeeze the whole thing.
Interesting. I think I had that gap too…I just used a fillet to help fill it (ha).
HOWEVER. Now I have a bigger issue late in the build. My galley hatch has a 1/4 gap from the hatch edge to the shell on the sides. I don’t have it all together yet, but his is going to be a major water issue I assume. No clue how to fix it….the ideas I have seen here (latches) all take away from the look of the trailer or I am not sure they will work.
So….do whatever you can to fix this now, try not to let the body expand outwards from the galley….which I think is where my other problems started.