We have one - LWB 220 with the high roof and it holds more then you think

have a 500l upright in it with a freedom cage. we fitted the tank in the middle so we still have access in the side door and the back. plus it meant we could bolt it through the chassis rails - 10 high tenile bolts, so that should hold it!
the cab is big - plenty of room for 2 adults.
i can email you some pics if you need any ideas of how to fit etc
Sean