Where I was to put the spare was probably the most difficult decision of the entire build.
I originally wanted to place it under the trailer just foward of the axle, but that would have severly reduced ground clearance as well as an increased ball weight.
Couldn't have it on the drawbar, no room left there after storage box and stone guard, so it more or less had to be on the tailgate.
I was concerned about puting it on the tailgate as I wasn't sure how
1). The tailgate hinge would handle it.
2). Whether or not the tailgate itself would begin to warp when opened
As you can see, it ended up on the tailgate out of necessity, but in hindsight, a good move, is accesable, counterbalances the load up front giving me a very reasonable ball weight.
Both the hinge and the tailgate itself have faired very well with no warping or anything untowards at all
As for how the wheel interferes with the camptop, it sort of does to an extent.
With out camptop, we have an additional annex wall which encloses the annex to the rear of the trailer. If we decide not to put the annex wall (75% of the time), then the spare wheel doesn't interfere at all.
If we do have the wall on, with the tailgate opened perpendicular to the trailer, the tyre does make contact with the annex wall (intrudes approx 100-150mm).
By doing so it stops the walls from sealing 100%.
The fix, well the tailgate is on the type of hinges that you just lift the tailgate up and off and then under the trailer
Or I guess you could remove the spare
Just beware that the weight of the tailgate, spare wheel and the additional bracing weighs a bucket load, I'd estimate around the 50-60kg mark, so removing the tailgate,not something the wife could do by herself.
Hope these help.