If I was going to set up a permanent site, I would begin with a good solid foundation to support the trailer like concrete pilings or pier blocks. The camper would be blocked up and the tires would be off the ground.
Next a vapor barrier and a layer of fine gravel, like crusher chips. Crusher chips are easier to level and will compact down to a fairly solid surface. One of the most important things is a good perimeter drain, you don't want water pooling under the camper.
Next comes a raised treated wood deck for storage under the trailer, then a 2X stub wall with foam insulation. If I remember my WA state geography, your still on the wet side of the state, so you won't need much heat down there.
The finish can be just about anything you want, once you have the stub walls in place.
What I've described is pretty much what you would find a manufactured home setting on. If snow birding is in the cards down the road using the 5er, I would change the foundation to a concrete slab and make the stub wallls in sections to allow for removal.
When I was a kid I was going nowhere fast, now I'm an old man I'm going everywhere slow.