[Motor31]

If you have deep pockets either New Horizon or Spacecraft will build you a rig to your specs.

You will likely need a truck with a higher load rating however.

If those are out of your price range then as hitchup suggested go to a few RV shows and see as many examples as you can. You'll most likely find that you can get most of the options you want but have to order the unit to get as many of them as possible.

First time buys are a real trial. It generally takes a few years of living in one to determine what you really want in a rig vs what you think you might want before hand.