When I looked in the factory service manual, I did not see any other tank size RPO codes listed; the 33 gallon tank may have been the only size available, at least in 1988. I do know the Suburbans of that vintage had 2 tank sizes available, which is what I may have been thinking in my first response to you. I'm not sure where an auxiliary tank or even a larger main tank would mount as there is not much unused real estate under these vans.