The "floating" fender thing isn't all that hard to do, but I wouldn't recommend trying it with fiberglass fenders ... 'glass just won't hold up to the constant abuse that is inflicted by being mounted to the axle/spindle. Those that work are all welded and internally braced steel, and typically mount to the brake mounting flanges on the back of the spindles behind the hubs.
I've done a sliding axle before, for similar reasons that you have going, but I planned for that in the wall framing and left adequate material in the walls to mount the fenders after the final axle/spindle CenterLine had been determined.
I'll echo what Woodbutcher and Thomas said above regarding potential fender interference with the side door(s).
Have you looked at the vintage Jim Dandy Sportsman
front-kitchen design for reference in your design? I'm pretty sure it's up in the Design Resources section of this Forum. It's fairly easy to see how the axle placement affected the side door size & placement. And, this brings up something that I'm not sure has been discussed, yet ... door-to-front-wall/bulkhead orientation and ease of entry/exit from the laying position. Now, I'm a big guy (6'4") with a bad back, and I've managed to squeeze myself into some mighty small teardrop interiors, but over the years I've determined that the minimum amount of room I need to comfortably
sit up and spin around to exit the trailer is 16" from front wall/bulkhead to the forward edge of the door ... otherwise I'm squirming & scooting to get lined up and out of the trailer. Some folks might require more, some less, but it should be a reasonable consideration during the design of our trailers ...
Good Luck with your build, and as always ...
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