Yeah, ain't 'imperial' units a bummer? 'Course, you haven't to call them that round Merkans - they call them 'English' units......
If you want your trailer wheels to follow in the tracks of your Samurai, I'm guessing you want to use Samurai-sized tyres - 205/70R15 if you haven't changed them for something else.
But if you're not driving over big rocks all the time, you might want to consider using small wheels/tyres, which can still fit in the Samurai's wheel tracks but provide you with much more interior space.
I reckon if you put 205/70R15 tyres on a 1300mm track (like the right hand half of the drawing above), you're going to get about 990mm (39") inside the wheel wells. If you are really narrow in the hips, that might be OK, but otherwise you're going to have to move the bed up in the air, so it fits over the wheel boxes.
If you put 4.80-12 tyres on a 1370mm track, their outside edges will be in line with the Samurai's tyres, but you'll get something like 1140mm (45") between the wheel boxes which is tight, but bearable for sleeping.
Of course if your mates will laugh at you for having small wheels, then it's not going to work, but I reckon the 235mm or so under an axle on 12" tyres is about what you've got under your Samurai diffs.
Andrew