2000 Bigfoot Camper 25C10.6B fit on dually longbed?

I would say the camper was made for the truck. Camper manufacturers tend to build campers for the trucks that are new at the time. It's better for sales than building campers that don't fit current model trucks. 1999 truck and 2000 camper, a match made in heaven.

You may need dually brackets for the front jacks to clear the fenders, but other than that...