This comes up from time to time, but I'll do the old song and dance again.
How much you can cantilever your walls and floor really all depends on how well your bulkhead and end walls are tied in to your side walls and floor. On a camper that overhangs the frame rails and covers the wheels, it is actually the side walls that are holding the edges of the floor up; and the bulkhead(s), front and rear walls hold up the side walls. The floor only needs to be stiff enough to bridge between trailer side frame and cabin wall, which is generally a shorter distance than in between trailer xmbrs. The walls have the deepest vertical section, so longitudinally they are the stiffest beams in the entire structure. The trailer frame and floor have the shortest section, so they are the most flexible (if left unsupported by stiffening bulkheads, front and rear walls and cabinets). The walls, bulkheads, cabinets (if made integral as stiffening structure) and roof (yes the roof) stiffen the cabin structure, stabilizing the walls so that they don't buckle and can stiffen the trailer frame.
Just making the floor thicker out of solid plywood is a waste of weight. Are you even going to be walking that close to the walls? Consider building the floor up out of thinner skins and foam core; the depth of section adds stiffness by pushing the outer fibers further apart; the outer most fibers (the skins) take most of the tension and compression loads from any bending, while the middle fibers don't do much but hold the outer fibers apart, which the foam is suitable for and much lighter. In a built up floor, if you will be walking on it you probably want the upper floor skin to be a bit thicker, maybe 3/8 inch, but the under side can be thinner to save weight. If weight is a real concern, consider using the thicker ply only in areas that will actually be walked on.
If you want to cantilever off of the front of the trailer frame it would be a good idea to have a forward cabinet with structural face frame, or a low shear wall built into a bench face/under seat storage bin, etc. Same thing in the rear, if you aren't doing a TD style bulkhead, add shear walls under your bed frame, storage lockers, upper cabinet face frame (think trestle) or a structural arch to bridge between the walls. You are not building a stone pyramid where everything gets stacked on top of the thing below it, resulting in a massive structure; you are building a bird cage Maserati (a classic race car that had no traditional chassis, just small tubes welded in a structural array of triangulation) where the sum of all of the small structural parts tie together into a very light yet rigid structure.
With a properly designed cabin you can cantilever quite a bit without having to add any outrigger structure at all; up to 1 foot is pretty common.
I recommend that you read a few more successful build threads of similar size to your project before making any decisions.