My rig has a 24ft box and the plan was simple, blow up air mattress, Loveableloo type composting toilet, cooler with ice with the goal being good enough for sandwich making, or eating out.
Suspect this might work for you, also. Once you determine you're really, really, really going to do this, then you can move beyond that. We ended up with LED lighting, an onboard generator, but never outgrew the inflatable mattress of composting toilet. Beats a tent. Also depends on your use case. For me, it's because I fly model airplanes and like attending weekend events so overnight Friday and Saturday, depart Sunday AM and life is good. Probably work just fin if you like going to the races and camping in the infield. Ditto some light boondocking. As regards full-time RV living? Nope, probably not such a great solution - but - it also entirely depends on what you need in the way of comfort, and whether you're single or equipping for a family.