The design of the Classic lent itself well to fitting a very unusual design of LPG tank underneath... a trapezoidal design tank. But that design was almost uniquely used on Classics, high cost and unusual to fit even on Classics. As has been said, most Classics had other tank options fitted, mostly cylinder tank in the luggage area or vertical toroidal tank in the spare wheel location.
On a P38 the biggest size tank that will fit flush in the spare wheel well is 720mm x 270mm. Such size tank will be rated at between 84 and 95 litres, the lower figures for 30 degree hollow centred designs, the higher figures for zero degree or 4 hole designs. It is possible to remove the bottom of the spare wheel well to fit a deeper tank (more than 270mm and 720mm wide tanks are made up to 350mm deep) but the P38 design has a rear antiroll bar close to the spare wheel well which cannot be fouled by the fitting of any deeper tank, hence most tanks in P38 spare wheel wells will be 720x270.
On P38's a second tank can be fitted in the luggage area. Such tank can be made removable using (e.g.) hanson fittings. I don't own a P38 but my Elgrand has a 720x270mm 95L 4 hole fitted to the external spare wheel position and a couple of hanson fittings that extend into the rear luggage area - currently I have an 80L cylinder tank hooked up to those hanson fittings and I refuelled with 175 litres of LPG today. When I no longer want the additional cylinder in the boot fitted I will very easily remove it just by disconnecting the 2 hanson fittings. My 720x270 95L 4 hole has working capacity of 95L because I modified it to fill to 100% instead of the usual 80%. My 90L rated cylinder fills to 80L.
When you have the ability to add a 2nd tank inside using quick release gas connectors you're not just limited to adding that single extra tank,. I could add any number of extra tanks and carry them inside, switching tanks as easily as switching between quick release connectors. The permanently fitted 720x270 external tank gives me 300+ mile range, the 80 net litre cylinder gives me around 300 miles on top, I could easily add a 3rd / 4th tank. Just with 2 tanks fitted I can tow my large caravan 500 miles without needing to refuel. I have thought about removing the petrol tank to permanently fit an LPG tank in it's place - which might (combined with just the 720x270) give range on LPg of 600 miles or 900 miles with 1 additional tank.. With that kind of range on LPG you don't need the facility to be able to run on petrol.