Do you already have a refrigerated truck body? If not, why are you choosing that particular structure?
Is this for a home that needs to last a long time (eg the rest of your life), or is it more something you need for a specific period of time? How long?
There are issues with metal bodies, but plenty of people do housetruck conversions this way. You don't need insulation (it's already in the structure). I'd go for a full size woodburner (depending on how long your deck is), and easily opening windows that are designed for good cross ventilation (and in a rainy climate you want windows that open horizontally not vertically). I have friends who built a housetruck on a refrig body, and that's all they did. They lined the inside and outside with plywood and timber, and had a woodburner and windows. It was very comfortable to live in (no big condensation issues in the actual home), but it's not as wet here as where you are. The issues in that kind of build are what is going on within the walls that you can't see.
You can do some research on shipping container conversions that use refrigerated containers. Condensation happens where you have warm moist air moving through the lining and hitting the cooler metal. What's going to happen to that moisture and any materials it comes into contact with.
Off the top of my head, you probably need to create a space between your inner and outer linings and any metal. There is a bitumen tape that you can put between any timber and any steel/metal that will limit the timber rotting from condensation. All these things are design issues that need to be worked through, and will be resolvable to varying extents, but
IMO are complicated and possibly costly if you want to build something that will last as long as a house. This is why one reason why I chose to build my housetruck from scratch rather than using a refrig body (I also didn't want to live inside metal).
In terms of
permaculture design, I think there are a couple of issues here. One is whether such a body is suited to your climate, and if it's not then you are going to spend a lot of time overcoming that both in the design and build, but also in the living in it (eg I would choose woodburner/windows over a higher tech heat/air system, because I like resiliency and simplicity, but it would commit you into having a steady supply of
firewood). The other is working with what you have (if you already have the truck). It really depends on what you are intending to do with the building once done.
That you have lived in a metal caravan for 3 years is a great resource, you will already have a good feel for what works in your climate and how to manage a woodstove in that situation.