It's a conversion that works very well for us, April. We spent two weeks straight camping in the trailer and found it was comfortable and functional. We'll experiment with different table shapes for a while before we settle on a final size shape and size, though.
One thing I do need to warn you about is I would NOT move the fresh water tank to the back of your conventionally towed trailer! The 5th wheel has enough tongue weight
on it that 140 pounds of water (a full 20 gallon tank) at the back of the trailer won't cause instability when we pull it. That's not true with your bumper-hitched trailer, but you can still turn the back bench into a space to tuck lightweight stuff, like clothes and such.
The table width was 30" bench-to-bench, and it's still 30" bench-to-bench. The 24" depth when added to the back bench seat isn't quite the same length as the original table. We made the table a tad shorter to help make it easier to slide in and out of the dinette. We'll probably make another, even shorter table because it's still a little bit of a chore getting up and down.
If you look at my earlier posts there's one where I show the "filler" piece that tucks over the "nose" of the table to fill in the gap when we want to turn the dinette into a sleeping spot.
The bench depth . . . I wish I could remember.
I think it's 16 inches, but the depth was more or less dictated by the size of water tank's 12"x32" footprint. The back wall of the Scamp
arches away from the tank from there by a few inches and the tank cover/bench extends a bit further forward, so 16" is probably about right. I'll try to remember to get the measurement this weekend when I go visit my trailer.
As long as you don't go overboard (like I did) this makes a good "starter" project for your trailer, April. Just concentrate on making the back bench with storage underneath and a new, smaller table with a podium leg. Don't remove and cut holes in the benches, re-plumbing everything, cut hatch-holes in the sides of the trailer . . . that would be too much for a first mod, and you'd loose use of your trailer while you made the changes.
Trailer mods are evolving projects. Do one little corner at a time, changing the way just one part of your trailer works while leaving the rest alone. That way you don't get bogged down and locked out of camping in your trailer for months and months at a time. That, and by leaving the existing bench structures intact you can play with the dimensions of your new "U" dinette and table and still go back to the original configuration if you don't like it.