Greetings!
Sofa/beds can be pretty easy to make, but comfort is a highly personal thing. For my current rig, I opted for a house style metal framed futon sofa/bed, then created the cushions out of 4" camping pads. It’s not quite as comfortable as my home made ones, but it was quick, cheap, and easy. In fact I got the frame for free. These also come as bunk beds if you have enough ceiling height.
I just extended the legs to clear my wheel well. The seat angle of the sofa is important to me, in the bunk picture, you can see the angles of the seat back and bottom. These do come in different designs too, make sure you get the wall hugger design.
My DIY version was actually a little more comfortable, and it was based on the spring bed frame design like this.
These used springs all the way around, suspending like a piece of metal fencing. So I built 2 matching frames out of 1" metal conduit, using the slightly smaller premade rounded corners. After adding the legs & stupidly simple leveling arms, I wrapped the top of the frames with boat rope (moisture proof), to create a way to attach the springs to the conduit frames. Then instead of metal fencing, I made a similar grid pattern out of the same rope. So this gives you a light weight, vented, spring suspended base under your cushions. VERY COMFORTABLE!!! If you decide to go this route, I can help you with the design of the moving parts for converting it between bed & sofa.
Cheers!