I laid down in the back before. it was nice.
Only short time though no long term camping or homelessness
BUt as i am heavy I feel as if I am breaking the plastic a bit.
for a regular sleep arrangement definetly a sheet of plywood to remove high pressure points that could crack the plastic.
and a comforter over it for ym comfort.
On long winter drives i always pack 2 or 3 comforters just in case.
(but not plywood sheet yet)