A trickle charger probably wouldn't keep up with much, I just thought of that when you said you didn't want to disconnect the battery and open the caps. Sounds like you'd be better off setting up an external power supply that you can switch to when plugged in just like a caravan. If you wanted to automate this and be able to run it via batteries seamlessly, maybe a computer UPS? They take 12V batteries (alter it to use yours instead of the tiny one inside the unit) and include the inverter as well as spike protection etc. Plug in the power and it charges the batteries and runs the appliances, remove it and they run via battery without switching over or having to turn off. You'd want to disconnect the warning beeper though, it would piss you off!
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