I've been pondering some of these same things,
50 amp plug even if you only need 30 now
14 door hieght
Doors front and back for air flow
Floor drain with enough grade so water actually falls to the drain and doesn't puddle
Long enough if possible to leave the truck hooked up if possible
Wide enough to open all the slides and still walk around/ work on camper
I would skip the skylights, wanting to keep as much UV etc off the camper as possible
Good luck and even if you decide to build your garage 100 X 100 eventually it will be to small !