AGM batteries (absorbed glass mat) are supposed to hold their charge for up to a year, I have a set of them.
Even so, since we don't use our trailer regularly during the year, and since it is stored away from our home, I remove the batteries after each trip and bring them home. I put them on charge for maybe 8 hours every month or so.
As far as battery switches, I installed these on our trailer .......
- Harbor Freight Tools - Quality Tools at the Lowest Prices
Cheap and easy to use - just turn the green knob a turn or so and your batteries are disconnected and cannot drain through parasitic loads from the trailer.
If you do buy a trickle charger I would strongly suggest getting one of the so-called "intelligent chargers" that will not try to continually charge the batteries. They cost more - maybe $40 compared to $15 for a straight 1-2 amp trickle charger. "Battery Tender" is one brand name.
The main reason I put the disconnect knobs on our trailer wiring connectors is that I don't really trust the charger built into the AS not to overcharge the coach batteries when we are on shore power for a long time.
Whenever we stay at an RV park more than a couple of days, I disconnect the coach batteries - need to remember to connect them again when we depart!
It may not be necessary but I felt it might be worth doing - and its a whole lot cheaper than replacing the AS converter/charger with a better one!