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.