If you really are considering wood..............................................
From a lifetime of construction experience and knowledge, as well as the first hand experience building my own true 4-season camper out of the material people have nightmares of...
I strongly suggest you stick to VG (vertical grain) fir for framing, completely encapsulated in epoxy, true BS1088 marine grade ply for exterior skins, glass, & epoxy.
VG Fir for strength-weight and rot resistance
Marine grade ply for its high ply strength, boil proof glues, and rot resistance
Glass and epoxy to blend joints and create a candy coating, then topcoat with your choice of paint.
My personal feeling is that anything less will result in the reasons people are scared to death of wood.
Start with quality wood products, do what you can to keep them intact, and you have a chance.
Mine has worked out beyond great. Its the equivalent of a true 4-season 10' slide in camper that weighs just 2200lbs dry.
Originally built in 2013/14 and until the last few weeks (in the shop torn apart for a refresh), has never seen the inside of a garage or even a cover.
It stays outside 24/7/365, and is no worse for wear.
Let me know if you'd like more info.