My Class 5 wheelbase with 60" CA is about one foot longer than a Class 3 with a 48" CA. The Class 4 is kind of a mess since there is a cab chassis and pickup model made by Ford. The Class 4 cab chassis version has similar dimensions to the Class 5 but the frame may be slightly thinner, while the Class 4 pickup is closer to the Class 3 in dimensions.
You can get the longer 84" CA model to add additional storage between cab and camper, but now you are adding at least another foot to your wheelbase and two feet to your overall length. I could see doing something like this if you are trying to eliminate/reduce the rear camper overhang by shifting it farther forward, but now you are sending more money on customizing the bed or camper so it clears all the outcroppings on the underside of the camper.
When you look at the current Class 3 DRW trucks, they easily weigh 3000-3500 lbs on the rear axle when empty. Once you add a 3-ton camper and some accessories, you have hit that 10k RAWR and we're not even discussing towing behind it or carrying a motorcycle on a rack...
Your 10' utility bed adds considerably to the rear overhang of your current truck. I would not be surprised if you are over 4000 lbs on the rear before you add the camper. If you have 48" CA on your truck with a 10' bed, that is a lot of rear overhang which does not help you. You're a little better with a 60" CA - I stayed with a 9' deck.