Truck campers have evolved over the years. Floor space is sort of a relative term. All campers sit between the wheel wells in the bed of a truck and early models that is the floor space minus the framing. In some newer campers the floor is over the wheel wells adding some floor space. Some now are over the bed rails.
Slides add interior space but not space to walk, but maybe a space for a dog to lay.
In general I think you will find that modern slide truck campers will have more space for your dog than a van. Probably a bunch more if you consider being able to walk around yourself. Larger campers with slides will need a bigger truck to carry it.
We have 2 dogs that are each about 55 pounds and they seem to have no problem finding places to lay about and us to move around.