disconnect battery or not ?

Disconnecting the battery certainly couldn't hurt, even disconect both. They may still drain slightly, and wont hold charge for ever, but doing this will help. I beleive a better alternative, the one i go for, is a trickle charger. They come in many shapes and sizes, and from solar to wall outlet, i think these are brilliant, and i've never had a problem, or a dead battery for that matter.....well from leaving the 4by stationary anyway! Hope this helps.

If you have the 2nd battery under the bonnet like I do, and I am not travelling with the fridge, I run my CB radio and air pump etc off the 2nd battery so it still gets some use.
I have these set up on a fuse box so I can easily swap them over if I want too.
That way if one fails I can run things like radios off the battery that is OK.
It is under constant charge via an isolator.

My 2nd battery is is a N70ZZ like the main battery, and I also swap them over every 6 months.

Just my way of doing things. I feel batteries like being used. If not being used, they need to be on a trickle charger.

Leave it connected, so it stays fully charged.
If you take it out, it will self-discharge and lead acid batteries like to stay fully charged.

thankyou for your information was much appreciated