I would think you could just drop the good one down to the minimum pressure, probably 5lbs, and you should be good to go. May not ride as good as you would like, but I don't think the world will end, without having the airbags pumped up.
Mine is only a 24, but we only run 7.5 to 10 lbs anyway. Yours is longer and may ride a little looser in the back end, but don't see how that would be a safety issue or stop you from going. Remember you still have the rear suspension the van came with.
Ps you would probably get more answers if this was in the class c forum