leisure battery to van battery

I have had BatteryMaster fitted on both my current van and previous and it has worked very well over more than 20 years. Fit and forget, it transfers charge anytime the engine battery drops half a Volt or so below the leisure battery, with the charging current limited to 1.1 Amps to avoid spikes, so will make up for any drain from e.g. the alarm system even when the van is not connected to hook-up. 

It's a very small, lightweight sealed unit which is very easy to connect and doesn't need mounting screws, I use a small Velcro pad to mount it on the side of the driver seat box by the leisure battery and relay/fuse block. It has a built in green/red LED so I can easily tell when it's transferring charge.

I have no personal experience of the Votronic unit but as far as I can see from the spec, the Votronic only works whilst the leisure battery is being charged: 

"Activation of the unit is automatic and it only operates when it senses a charging voltage at the leisure battery"

which means only when on hook-up (or I guess if you have solar?).

This may be OK for some but our circumstances mean that our van is sometimes left where hook-up is not available for long enough that the alarm would drain the engine battery to an unacceptably low level, so the BatteryMaster is the best solution for us.