I'm with gbw. It doesn't sound like you need a DRW with only 6440 on the rear axle. You can easily go up to 7500 on the rear axle with stock type higher rated wheels/tires.
You have to find out what your wheels are rated for, but you could possibly just change the tires. In any case, you should be able to find used F350 wheels that will be rated high enough. Then, you just need to make sure the tires are rated at 3750-4000.
You don't say if your camper will go on a long bed, but if it doesn't, the number of short bed DRWs is pretty limited.
