W/4 aluminum wheels & 2 steel, extensions are an unneeded complication IMO. The steel nipple for the inside wheel should protrude neatly in the middle of one aluminum wheel hole (and you should add a rubber grommet centering it, if you don't have one) if the duals are properly rotated.
On my last coach w/dual steel wheels & covers, extensions were an evil necessity, but once they were on properly they worked flawlessly. Seems I either pinched the o-ring & it leaked, or I didn't and it sealed perfectly for life.
Extensions, BTW, may have a tire/wheel balancing req'mt if added to an otherwise balanced wheel.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project