If you put cabinets along one side wall, against the bed, don't put extruding handles on them or one of you might hear some new words emanating from your spouse's mouth in the middle of the night.
I have friends who complain about beds that lay sideways in the trailer, complaining about the "yoga" that they have to do to get out of bed when they are the one up against the back wall. I, personally, have no experience with sideways beds in trailers.
My bed is lengthwise and I crawl down to the bottom to get out. In about six thousand miles of camping, it hasn't caused any problems for me yet, even when the dog is sleeping up against my side and my other side is against the wall. The one possible problem that should be considered is that, when you get out of a teardrop, you can roll to your side and push with your hands to get up. When your bed is lengthwise in a standy and you need to get out at the bottom of the bed, your arms and legs will afford you little help. Unless you have good torso muscles, you might plan to add an extra piece of wood framing for backing a reachable handle on the side closest to the wall and a handle on the cabinet (if it is a floor to ceiling model) or something like that, to assist you in lifting your body out of the bed, should you find you need it later.
You can get away with a smaller width than the standard (39") twin size, using, instead, roughly half the size of a full bed (54"), which would be about 27" or maybe a little more, say 30"(?) and making your own mattresses out of foam, etc. Many of the older Scotty-type trailers had "full" beds that were really only 44" wide, which is not a standard full bed width for two people. If you decide to go with "twin" sized beds, I'd put the aisle in the center. With twin beds, you would have the added advantage of it being easier to make the bed when you get up in the morning but you might miss the shared body heat on a cold night.