Ken,
You might want to take a look at the vintage archives picture site here
http://www.vintageairstream.com/arch...756Bubble.html
Most of these seem to have come equipped with one tank. There looks like there used to be a plate between the frame rails that acted as a platform for the small tank. Some of the units pictured also seem to have a basket for the tank. If the plate is still there then it is just a matter of fashioning a mounting system. If the foot of the tank has holes in it you coudl just clamp it to the steel plate using threaded hooks and bolts???
You may wnt to wander on an RV lot an look at how the single tanks are mounted on a pop up. Silver spray paint and no one will know the diff.