I would completely skip the 2500 truck and look at a 3500. The weight of the camper will likley be a minimum of 2,000 pounds even without a bathroom in it. By the time you add the tongue weight of your boat and the 3 to 5 adults you mention in the truck you will quickly run out of payload.
You can most likely tow with no extension behind most of the 9' campers. The use of an extension isn't that big of a deal in the smaller sizes. It can actually be helpful because you don't have to get the truck as close to the water to launch the boat.
I have an 11' camper with no slides and no basement, therefore the floor width is only 48". With me, my wife, and 2 young sons it is very very cramped. Now that we have a 3rd child it will be going up for sale. I would take the whole family with you to look. Get everyone inside the camper and shut the door. See how it will be to actually be in there for a bit. It may be that you need to look at 11' campers.
I've towed a 4,500 pound boat behind mine many times with a 48" hitch extension and it works fine but I'm 1,600 pounds over the GVWR of my dually.