Battery charging

GeeTee
Unless you live somewhere exceptionally hot, the best way to keep gel cell batteries happy when they are not in use is to fully charge them every 6-9 months and then leave them somewhere cool.

As with AGMs, gel cells have very little internal leakage and do not in any case suffer from the sulphation problems on conventional batteries left uncharged.

If you really do want to float them they you need a top quality thrre-stage charger with a gel cell cycle. But my advice is not do this.

Of all the mythology surrounding batteries, one of the biggest is that they need exercise. The opposite is true.
Collyn Rivers

Grock
You certainly can insert a manual circuit breaker. Locate it as close as feasible to the battery. (Do make sure you choose one that can be manually opened (some can only be manually reset - get back to me again if you have any problems finding one). Use one that cuts out at about twice the normal full-load current.

The voltage drop from 12.9 to 12.4 over five weeks (for a 70 Ah AGM battery bank) suggests a probable total drain of about 20 Ah - or around 0.5 Ah/day.

This is massively higher than an AGM's internal loss. Suggested causes are a radio that draws slight current even if turned off at the set (this is common - some have a second direct battery feed for internal memory etc), an exceptionally large electric clock, an electric fridge that draws slight current even when 'off' - see below.

Many electrical appliances (including some 12-volt ones) have an RFI suppression circuit on the battery side of the on/off switch. The intent is to supress interference when the appliance is switched on and off. These typically draw current of the magnitude manifest here.

If in doubt always turn them off at the wall switch - or disconnect them from the socket.
Trust this helps
Collyn Rivers