Van, or B, loads used to be 2000# but as competition got wilder, some carriers would haul up to 2500# for van tariffs. Some carriers now call a van load as much as 3000#. Your currenet carrier of choice, Panther, I believe, allows you to determine, within reason, your weight limits up to 3000#. When they say between 5 and 6K, it means they don't know and are guessing.
I'm certainly no cube van authority, but my guess is that the extra weight of the cube and the dualies is closer to 6500 than the 5000 guess. Better get it weighed than be sorry after you build a sleeper and add a fridge, microwave, wall, cabinets, luggage and two people with full fuel tank, E-Track and all your safety eqpt, tool box and all the other stuff you already know you want.