I'm thinking that by the time you design and pay for a system that allows you to easily, efficiently and quickly move a 1,500-lb+ camper shell off a utility trailer, you're halfway to just paying for a new trailer already

A time-tested method for a removable box is, as others have suggested, a wheel-in-track system with the necessary structural reinforcements and jacks to make it happen. It's not super-complicated; just time-consuming. I like Toyer's idea to make the permanent mounting base the same height as your trailer bed.
5x8 does sound rather small for all those features. You'd have some crazy overhangs! Are you going places (e.g. off-roading of any sort) where you can't afford the higher center of gravity and reduced stability?