We have (an older) 13' Scamp.  Many years ago, we replaced the internally mounted 12 gallon tank with an externally mounted 20 gallon tank.  It is mounted tight against the floor immediately behind the axle.  Our purpose was not to provide more tank capacity (we rarely carry more than 5 gallons), but to create more internal cabinet space.

Some have questioned that this location might unbalance the trailer, but this does not appear to be the case and we have never had a problem.  It might be noted that the intenal 12 gallon tank is located behind the wheel well, and all the weight is on one side.  In our location, the tank is closer to the axle and the weight is distributed to both sides of the trailer.