I had Westfalias in Europe and both of them had bottles that were inside the van under the sink.  I had to attach the hose that had the regulator and the valve on.  
I prefer to have a bottle outside though.
The pipes that lead to the tank under the car can be cut and have a fitting installed with valves and anything one wants to.  Some guys have catalytic heater in the car.  One can have an extra bottle too.  
In fact the one I had in Europe had about five gallon in it.  I think, 25 liters.  
Zoltan