Hi Tom I believe its to keep the weight down, most hold about 2-3 imp gallons so your looking at 20 to 30lbs in weight. The majority of European?s are still built on 3500kg GVW chassis so payload is minimal to say the least, usually no more than 400kg's and that excludes driver, passenger, LPG and water so a holding tank as fitted to ARV's is not an option. There is also the size, most are no more than 21' in length and 7' wide no room underneath to put the tanks anywhere.
Fuel price's overhere are the overriding concern of most owners, petrol is about 86p per litre and diesel a little more, so the lighter you are the more to the gallon, most will do 23-30mpg. I had our Brave converted to LPG/petrol LPG it's about 38p per litre and gives me about 18mph petrol equivalent.
Its a bit like comparing a gulfstream executive jet with a Cessna.
Olley