Is this trailer to be double towed behind your fifth wheel or just behind the truck? I suggest matching the wheel and body width to the vehicle to which it is primarily attached for the best handling.
I tow a 20' enclosed Interstate Car Hauler which is 8' wide. I wanted the ability to carry toys or a car, so I have dual 5200 lb torsion axles. I feel the torsion axles are much easier on your trailer and load since each wheel can move independently of the others. I run three lengths of e-track on the floor and run four strips vertically on each long wall. I do not have an escape door on the left side, but I can get in and out of my vehicles without one. My right side door has an RV style latch allowing opening and locking from the inside.