Unattached beds, such as foldable cots or mattresses, offer the flexibility to completely stow your bed during the day. In cold weather, you'll need additional insulation (pad/blankets) under you too.
It's usually difficult to store things under cots, so that space may not be useable for other things.
You can also use a hammock that is hung between mounting points you've placed on the vehicle interior; hammocks may or may not be compatible with your body for long term use, so be sure to test that before making a hammock your only sleeping option.
With unattached beds, remember to keep crash safety in mind. The bed frame or even the mattress could kill or injure you during a collision, so be sure that it is well secured when driving.