I think the king bed requirement may be the biggest determining factor on what you purchase. They aren't available in every model, but it does seem like they are found more frequently in fifth wheels.
Length wise... You might encounter some campgrounds that you cannot fit into easily with a long rig. But, your driving ability can also determine where you fit. The more comfortable and better you get at backing, the more spots you can manuever into. I have seen my DH park in many places he was told he wouldn't fit. Just plan ahead where you are going to stop each night.
I would encourage you to look at a dually vs. a single rear wheel. You'd probably be glad to have the payload and stability the extra set of tires provides.
The Mega Cab Dodge has an enormous amount of space in the backseat, and now that we have one, it is hard to imagine how we functioned without one.
