Welcome to the forum and good luck with your build!
I think you may want to study and do a test mock up of that arrangement before committing to anything.
First couple of things that strike me are that a queen size bed is 80 inches long and you have only allowed for 72, not counting the thickness of the walls.
The door sill will be very high (nearly 3 ft by the looks of your drawing) and you may have a hard time getting in and out.
48 minus 16 leaves only 32 inches of headroom not accounting for the thickness of the subfloor, floor and ceiling/roof. Seems a bit short for an average size person to sit up in.
The basement idea is a good one, if you have lots of gear that you want to store, but if you don't remove the metal side grates how are you going to get to it? That's an awful lot of stuff to have to drag from under the mattress.
Some suggestions (my opinion):
Get rid of the metal rail and grating on the trailer. So long as you build a good rigid box and attach it well to the main trailer frame, it is not needed and will only get in the way, collect dirt and moisture, and be extra weight to carry around.
Think about your joinery; how are the walls going to attach to the subfloor, floor, ceiling and roof (that will give you a better idea about how much interior space is truly available and will also give you an idea of how many sheets of ply you will need... I'm betting that you will end up taller than 48 inches).
Figure out what stuff you need to bring, measure the big stuff and see how much room you really need. If the basement is only needed as a way to get up above the railings, then just get rid of the railings.
An option to the basement is to build a storage compartment between the galley and cabin that is accessible thru doors from either side. There is a build here somewhere like this, but alas, I can not remember the name. It was well done and I thought it was a great idea, so long as you don't mind going a little longer on the profile.
Hope you find these suggestions helpful, as they are intended.