Some people put it on the back bumper. I've never liked that because it takes away from the streamlined look. Also, it gets in the way if you have a storage compartment bumper.
Some people put it behind the propane tanks. I did that with my '67 Caravel, but it looked like clutter to me. Plus, the Caravanner has a battery (or storage) box up front so there's no room.
For one or two model years, Airstream created a wide storage compartment on the side. You slide it in from the outside and it rests in one of your base cabinets. Cool, but cabinet space is precious.
In my book, there's only one place to put the spare: under the trailer. It's a little tricky. You can see in the photo above that it requires another chassis modification near the front in the A frame section.
Having it underneath does mean getting under the trailer when you have a blowout, but it's in the front area so the clearance will always be good. The belly skin in this area will wrap up around the framing and directly up against the floor so there will be no room for insulation. The other disadvantage to this method is that the tire hangs down 5" below the plane of the belly skin. That bothers me a little, but we may mitigate part of that problem by raising the trailer an inch or so with the new axle.
Shown With Tire in Place