The first time I brought up the idea she thought it was the dumbest thing she ever heard. But I just kept bringing it up and eventually started saying things like "when I get my bus I'm going to...." and so on. You have to condition your woman into thinking that buying a school bus and turning it into an RV is something that's normal and all the rage.
Tell her that you'll need her help with some sort of project on the bus. Get her involved! It takes a bit of time. Just keep bringing it up and eventually, she'll let you get a bus so you'll shut up! That always seems to work for me.