Downside is you're limited to where Amtrak goes (or having to hire a locomotive and getting permission from Union Pacific, Norfolk Southern, etc. to use their tracks)

If I wanted to go the van-life route, I'd probably order a B-Box from Advanced RV in Cleveland. They have more interior room than a Sprinter and are better insulated. Costs about the same as a railcar and currently has a year-long wait.