I suggest getting a trailer that is as wide and almost as high as your Roadtrek. This will maximize space yet not affect wind drag and still allow you see it when backing. I prefer the wheels tucked into the trailer body instead of dealing with additional width and not getting that additional space inside (even though you have the interior wheel well obstruction).
As others have posted, a torsion axle will allow the trailer to ride much better than a solid axle plus you can often find these trailers with a drop axle configuration that allows them sit lower to the ground. This makes for a more stable trailer plus minimizes ramp and break over angles. A single 3500 lb torsion axle would work well for your your application, but you may want look into the 5000 lb version to give you some extra capacity if your needs were to change later. With either choice, get electric brakes for the axle so you have adjustability through the gain control and do not have to rely on the Roadtrek brakes for the additional weight.
Although you may not need a WDH to tow your trailer, consider one if the rear end sags on your Roadtrek. This will keep your current ride comfort verses adding on to the suspension and making the rear stiffer.
I run three lengths of E-track on my trailer floor and run vertical strips of track up the walls to tie down other loose items. Because I carry a variety of vehicles, I like the versatility of adjustable anchor points verses fixed.
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