PMF/Foamie Rolling Coffin Construction Technique

tldr; Can I build pre-fabricated XPS foam and poor man's fiberglass (PMF) panels and then construct my hard-sided tent on wheels, or should I construct first and PMF afterwards?
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A little background: A year ago I bought a small, used 4x6 Tractor Supply utility trailer that already had a four foot tall wooden box built on it. I turned that into what amounts to a hard-sided tent on wheels (or coffin, as my wife puts it). It's just a place to sleep with a little storage space for essentials (camp chairs, cooler, camp equipment). The box that came with it was not water tight, and in an effort to get it converted quickly and cheaply, I did my best to insulate and waterproof it. In October, we had just about a month of rain followed by some unusually warm weather. When I went to prep the camper for a December camp out, I found white fuzzy mold growing in the places that leaked the most. I pulled everything out and cleaned the mold, but I think it's time to just completely dismantle the thing and start over (something I've wanted to do from the beginning, but time and cost were against me then).
Even before I bought the trailer, I'd been looking at builds here on tnttt, and foamies really caught my attention, especially since it's a small trailer I wanted to insulate the camper for winter camping anyway. I've read/watched much on PMF, and it appears that folks do all construction first and then skin the entire thing with PMF afterwards. Is it inadvisable to build the individual sides/roof complete with PMF first and then put it all together? (My plan is to use 2x2 or 1x2 for framing and fill with XPS sheets.)
Why would I want to do things that way? Mostly due to space considerations. I have space indoors to store panels, but not the full sized camper, while I work on it. I would also like to still use the camper as is while I construct the new one. Obviously, my main concern is preventing leaks at the panel seams and corners. Would glue and screw be enough? Perhaps glue and screw with weatherstripping/butyl tape?
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A little background: A year ago I bought a small, used 4x6 Tractor Supply utility trailer that already had a four foot tall wooden box built on it. I turned that into what amounts to a hard-sided tent on wheels (or coffin, as my wife puts it). It's just a place to sleep with a little storage space for essentials (camp chairs, cooler, camp equipment). The box that came with it was not water tight, and in an effort to get it converted quickly and cheaply, I did my best to insulate and waterproof it. In October, we had just about a month of rain followed by some unusually warm weather. When I went to prep the camper for a December camp out, I found white fuzzy mold growing in the places that leaked the most. I pulled everything out and cleaned the mold, but I think it's time to just completely dismantle the thing and start over (something I've wanted to do from the beginning, but time and cost were against me then).
Even before I bought the trailer, I'd been looking at builds here on tnttt, and foamies really caught my attention, especially since it's a small trailer I wanted to insulate the camper for winter camping anyway. I've read/watched much on PMF, and it appears that folks do all construction first and then skin the entire thing with PMF afterwards. Is it inadvisable to build the individual sides/roof complete with PMF first and then put it all together? (My plan is to use 2x2 or 1x2 for framing and fill with XPS sheets.)
Why would I want to do things that way? Mostly due to space considerations. I have space indoors to store panels, but not the full sized camper, while I work on it. I would also like to still use the camper as is while I construct the new one. Obviously, my main concern is preventing leaks at the panel seams and corners. Would glue and screw be enough? Perhaps glue and screw with weatherstripping/butyl tape?