So your wife wants you to build a tiny house, so you figure that you will gut an old Travel Trailer, replacing the aluminum sides with ceader shingles, after the axels (sic) since she won't live in it if it looks like a trailer. I'm not sure how you insulate a two inch wall "as well as possible" or get plumbing ready for a Michigan winter - with sans experience on top of it all. And oh yes, you want to keep it mobile. What is the grand paln for towing this thing? Never mind that - how are you going to heat it? Please don't say space heaters.
If you don't mind, please share why you would choose to embark on this sort of venture.
If you really want a tiny house then google "Tiny House Movement" or maybe have Pete Nelson the treehouse guy build you a treehouse.
My best advice is don't try to live in an RV during a northren winter. You don't want to be found imitating a couple of popsicles. Good luck.