Wouldn't it be easier to just power the alarm from the habitation
battery?
In most cases no.
Thatcham's requirements state that "The security product must be installed to the manufacturer's specification". Now if the alarm manufacturer has stipulated that it has to be connected to the Cab
Battery and you change the feed, sods law this is found out and the engineer then could say it has been modified away from manufacturer's specification, then you could void your
insurance.
We also do not connect to leisure
battery as it can (especially with the very latest Mercedes and VW chassis) cause issues with the Can-Bus/Multiplex wiring harness for doing simple things like reading door switches, controlling lights etc etc. We have between 2018-2019 spent a fortune in R&D for our Security systems (including the latest changes to Thatcham's tracking listings), so that we can get and independent system to work alongside these chassis without relying on the Can-Bus to control (Which BTW is what other 3rd party/Dealers are doing as it's easier for them to do, sadly though seconds to by-pass).
If you happen to be using a factory fit system, chances are the feed cannot be altered anyway, as the alarm is part of the body computer so cannot be changed. However as the motorhome industry tends to use chassis that only have immobilisation, then retro-fit alarms are common place in the MH world, sadly though some using inferior Can-Bus/OBD driven units that are easy to fit, but equally easy to get round.
I cannot comment on other manufacturer's, but we also made the effort to have a system that has a very low current draw (less than a digital clock memory on the dash when armed up). It's worth noting that it is common knowledge that the X250-290 (Fiat/Peugeot/Citroen) chassis has been well known to have a high engine
battery drain, so most good systems won't be the problem to engine
battery drain, it will be a natural drain on the system which the manufacturers will do nothing about (other than Peugeot doing something with the ignition key, which disconnects most of the power from the van when engaged).
We manufacturered
Battery Master years ago, where it would combat vans that are parked up for durations, unused, with the van either plugged in, using a
solar panel, or even simply sat there with an unused Leisure
Battery as a resource of back-up power. At the moment, we average about 20-30 units supplying to dealers alone, not including our own fitting here.