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>buy station wagon with a good 190 cm or 75 in clearance with the rear seats folded
>throw mattress in the back
>throw bags on mattress, move them to the front seats for sleeping, move them to the back when driving
>profit
Keep width in mind. Non-American wagons usually have around 100 cm or 40 inches clearance between the wheel wells. American wagons usually have between 40 inches and 48 inches clearance between the wheel wells. This makes for a good fit with a 190x90cm mattress in Europe and a twin size mattress (75x38in, not twin XL!) in the US for one person. The big American wagons haven't been in production since the '90s, if you want something newer but as big or more offroad capable you'll need to get an SUV.
Everything else is optional, like a slatted bedframe with or without storage underneath, curtains or other things.