Horses for courses, there's no perfect solution that fits all scenarios.
Obviously if you're going to be using your MH every so often then disconnecting the batteries will be a complete pain in the ass but if you're the type of user that lays the MH up for 5 months then I can't see an issue. If the cab
battery was removed then you may well technically void your
insurance but really...how on earth would the
insurance company know unless you told them? I've had a MH nicked and was never asked if I'd disconnected the
battery. Flat
battery or no
battery? surely the same difference? Van probably safer and less likely to get nicked with the
battery removed.
If you're not plugging into the mains to maintain the batteries then the power has to come from SOMEWHERE, you can't get energy from thin air so the DC to DC maintainers will simply remove power from the hab
battery in a no sunlight scenario that I mentioned, if you have LA leisure batts they'll suffer. Nothing is cut and dry, especially when you have to take cost into consideration.
Just my opinion.