Sigma S30 Alarm draining battery?

We bought a Swift Suntor RS590 motorhome last year and at the end of the season put it into storage, we had only been out with it twice.

This year due to lockdown and a couple of other reasons we only got around to visiting the MH last week, I thought that the engine battery would be flattened due to lack of use so I took jump leads too. So I wasn't surprised when it wouldn't start. However I couldn't get it to start with the jump leads either which were attached as per the Fiat Ducato user manual i.e. under the bonnet.

The battery itself is in the passengers compartment, under the floor so I removed it and took it home to recharge then I found that it wouldn't charge on my charger so I bought a new battery (and a new charger as the one I had had failed to charge a couple of batteries recently). I fit the new battery and it did start - hooray! I then removed the leisure battery to charge at home, I also fitted new batteries in the Fiat key and the Sigma remote fobs.

Yesterday when we went back the engine battery was flat but did start with the jump leads fitted directly onto it rather than on the recommended point under the bonnet. There is more to this story but I've rambled too long and all the salient points are here.

So my question is this:- has anyone else had a similar experience? I made sure that there was no other sources of drain on the battery which leads me to think that it is the alarm that is flattening the engine battery even if it's only over a couple of days - is that normal?