deminimus,
This is a quandry for all of us truck camper types. When I read your post title, I thought, before opening it, what a great idea putting the second spare on a 2.5" square tubing rack inserted into your class V hitch hole and tucked under the rear of the camper.
After actually reading your post, I still think it's the best place to store a spare, even with the reduction in departure angle. I think you could get the clearance very tight and tire up as high as it will go. The weight is down low. Easy to get to. Does not restrict air to the radiator, or cut down on your view out the front.
There are just not a lot of options on a second spare placement. I certainly wish I would have had two on our 'to AK and beyond' adventure a few years ago. I used 17 tire plugs to try to stem the flow of a massive tread separation on a BFG AT BAJA to no avail. We had driven 2500 miles on gravel/dirt roads by this time. Not having a 2nd spare changed our route plans. Here we are gravel pit camping at Coldfoot, AK with the Brooks range (and north slope) in the distance:
Another way to go would be to take one spare tire tucked up under there with no rim. You can always get some local to mount it for you, if needed. It would save some vertical space and about 25 lbs. with no rim.
On a trip to the deepest heart of Baja about 30 yrs. ago, I took THREE spare tires/wheels along knowing how the volcanic rock on the east side eats tires. Well, it ate all my tires, as we had seven, yes count 'em, seven flat tires, several were completely unfixable. Drove the last 100 KM north into Puertocitos having to stop every KM to inflate what was left of the tires with a pair of hand pumps. Oh, those were the bad old days. Only fun in retrospect.
That was in my Chevy powered Toyota FJ-55 Land Cruiser with ******** cepek roof rack that held all the tires.
regards, as always, jefe