My camper is about 8' wide. So, sideways doesn't work unless you are under 8' tall.
That said, I once owned a Bigfoot 5th wheel, 20' long. the bed was in the overhang, which in this size 5th wheel was a lot like a camper, just a bit taller. The bed (a true queen, not a "travel" queen) was sideways. Enough free space at the foot to get by the other person when you got up. That 5th wheel had the best arrangement for a camper, bar none. If I ever build a custom, it will look just like that.
On a (marginally) related note: in the big long distance racing multihull sailboats they always sleep feet forward so that if they hit an iceberg going 30 knots you don't break your neck. Might apply to camper if someone sleeps underway.