on the outside, vertical lines of rivets will tell you where the ribs are. There won't be any ribs in the way. If there is a rib-like channel which frames the current vent (a u-shaped piece of thin aluminum sheet that is between the inner and outer skin), you'll have to take out its rivets and remove it, then you'll be able to look between the inside and outside skin to detect any wires (pushing back the insulation, of course). Generally speaking, you won't have trouble with wires in the walls, particularly behind where the old water heater is installed.
The newer water heaters sit right on the floor, so you'll the wood floor exposed right up to the edge. The banana skin will come right up to the flooring, perhaps overlapping a little. Once you've cut the right sized rectangle in the skins, the heater will slide right in. It attaches with 12 screws. Get some butyl tape (like plumber's putty, about $5 in most RV places) to apply between the heater flange and the outside skin.
You won't need the rain drip edge that is currently above your existing vent, but if it's at the right height, I'd leave it. Before you slide the heater in place, you'll need to drill a hole in the floor for the propane line, and a matching hole in the banana skin--there's a hole provided in the inner face of the new water heater to pass this line up and connect to the heater control valve. I've always found this connection to be a pain. You can put the hole in the banana skin back under where the skin is horzontal--with the appropriate curve in the propane pipe it'll come out of that hole and fare along the bell pretty closely.
Put some type of seal between the bottom edge of the heater face and the floor--butyl tape or vulkem. Then the only water path to the inside would be through the propane pipe hole in the face.