Where can i install a GAS IT tank on a vehicle?

The normal place to fit a GAS IT tank is underneath as its a win win location because it frees up internal gas locker space and also put all the high pressure gases outside the vehicle but in reality it is legal and allowed to be install anywhere you want inside the vehicle.  It doesn't even need a gas locker making to cover the tank because every GAS IT tank comes with its own, legal and certified gas locker in the form of the back airbox covering the valves.

Just use common sense when locating a GAS IT tank inside the vehicle so the tank can't be damaged or the venting pipes knocked and broken (which the black airbox does need to be fitted with to exit down and out of the vehicle)  by repeatedly having things put on top of it.  Most importantly also make sure the place you are physically fixing the tank to can withstand the legally required R67, BSEN12979 and EN1949 requirements for crash ability. eg the fixing you use and the place you attach the fixings must be able to withstand a forward 20g and horizontal 8g force.

As all UK Motorhome manufacturers have confirmed to the N.C.C that thier factory built gas lockers will won't be capable of withstanding those 20/8G required forces to fixing a R67 approved gas tank or gas bottle in a gas locker, even with all the right metal straps and fixings, will not comply with the regulation needs of 20G or 8G.