Different options
I have a voltage sensing split charge relay. Its connected to my vans 2x 80 amp batteries. I use a Halfords 110 amp battery. Quite impressed so far.
The problem is, It kicks in at 12.6 or 12.8 volts. So its always connected really. If you charge the leisure battery then put in back in van, it will charge the van battery up as it will be above 12.6 volts. It works both ways. If you could isolate from van until van alternator was on, it would be ok
Ideally have a battery same amp as van battery.
You can get a massive marine battery or large leisure battery. Should last you much longer, but more to lift about.
You can get a better alternator.
What van is it