I had a similar experience with a van at a Haven site. It seemed to get through 5 - 6 of the large propane cylinders during the summer season, which worked out at about one per letting. I used the same cylinders at home (no natural gas at home) and used 2-3 per year with gas cooking and fires. Makes you wonder what happens to it doesn't it. I reckon that either the site is fiddling or someone is swapping their empty ones for your full ones
We used to own a park home which used calor bottles and we used one a month but that was for heating as well, didnt use much in summer though.
We have just made inquires about owning a static and stopping touring and was told that the average consumption was 10 bottles a year and about £52 a bottle, well you can work that one out yourself!!!
We use about the same as dreadnought. 1 large propane bottle lasts about 6 month thru the summer and my wife usually has the fire on each night, we have a gas cooker and we both have showers, so your usage seems pretty excessive.
Yes definately lock your bottles, but make sure the site owners have a key in case of emergency when you are not on site. We used to have a static and had bottles swopped a couple of times - hence the chain & lock
------------- Enjoy life whilst you can, it doesn't last for ever!!!!!!
we used 2 13.5 kg cylinders last year over around 10 weeks, showers/cooking but no heating. we use a parafin heater to heat the van - NO i am not mad! its called an inverter at 160 euros to buy - fully controllable, timer to come on before we get up and it doesnt smell! i wouldnt have one though if we rented the van out !