Iain,
removing the large tank is not a job I'd relish. If factory fitted it should have a couple of straps over it, but look down through it and you will probably be able to see a significant amount of mastic squirted between it and the caravan floor that glues it in place. I think you would need a cheese wire to get through this.
You might however be very lucky and find your tank was a retro fit and held in place just with the straps.

Colin