Help request from a newbie.

Getting to know my recently purchased Reinza but are struggling with a few issues. The hand book supplied with the van is of little help and in some instances no help at all.
  Water. I presume that this is filled from a filler cap on the offside extreme rear, Does anyone know what capacity this is?
Also the lockable cap has the key missing. There is a number stamped on the plastic on the inside of the filler cap, will this be the lock No?
 When I have filled this with water it is being dumped from a small tube under the offside almost exactly over the rear axle.
       I have poured 5 litres of water down the sink and the waste is not registering on the control panel. Is this a fault? 
   Gas. does anyone know what the largest gas bottle is that fits in the gas locker.
  Fridge. This seem to cool on 240v and 12v (with the engine running) but it is silent in operation,(the fridge, not the engine) should I hear this working? 

Any help, advice, or tips  would be greatly appreciated.

Hi fellow Rienza owner. Right firstly the water tank is 60 litres fresh water, waste water 56 litres. I can’t help with the locking filler cap apart from ours being a different key to the locker and habitation door, same type of key with a round black fob. If yours isn’t locked I wouldn’t rush to try and get a key. 
The water being dumped over the axle sounds like the boiler drain tap is open. It’s located under the o/s bench, it’s a blue tap. 
The level gauge is for the fresh water only as far as I can remember. There is a light that comes on when the waste tank is nearly full. Ours has never worked. Equally the fresh tank levels are not to be trusted. This is a common trait. Work on two days between fills and emptying.
The largest gas bottle is 6 kg. They are a bit of a finger trapper especially the back one. 
The fridge is silent on all settings. To be sure ours is lit on gas it will tick as it ignites and you can feel the vent outside gets slightly warmer. On 12 volt when the engine is running ours seems to cool surprisingly well. Don’t put ice cream in the freezer and then out for the day without switching over to gas or mains. 
Any further help just ask. After six years I know most of the Rienza traits . Lovely motorhome.

I should think it would be easier to get a replacement locking water filler cap than try to match the key. Perhaps it looks like one of these?

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Thanks , this explains a lot. I will try to find a key to the water filler lock there are a lot of pranksters about and I would not feel comfortable with the notion that anyone could interfere with the water.