Can I give some of you guy's a little tip when fixing tanks to your floors??
Bolting direclty to the floor even with plates underneath is not 100% safe.
but the biggest tip I can give you is, even if you bolt it to the chassis, you should not just drill a hole and put a bolt through it, the chassis metal is to soft, the correct way to do it is to drill a hole bigger than the bolt you are useing, through the van floor top and through the top section of the chassis, but not through the bottom section, the hole in the bottom section should be the size of the bolt you are useing,you should then drop a steel tube that just fits your bolt, into the hole, this way when you tighten the bolts you wont crush your chassis and this also creates a stronger fixing.
The tube and the bolt holes should all be a reasonable fit and not a loose fit.