On NY9D’s trailers, we are setting them up like phone company central offices. These buildings run all equipment only on 48V batteries. Some modern datacenters do this. The AC power from the power mains or generator only goes to the battery bank via chargers.
In the trailers, we use 12V, and they have one, two or three 12V batteries, usually Group 27 deep cycle. These are sturdy and cheap. (Best to not mix battery types). You get about 50 usable amps from each of these (1/2 name plate capacity). But they are resistant to over charging and deep cycling (mostly) and can start an engine. They are heavy and can spill acid- but not on a big trailer. Nicer ($$$) batteries are of course available.
The use cases are around solar or shore power or generator backup. I experimented with some $15 4AMP “smart” battery chargers. These had ring terminals for permanent attachment. The ones I found you had to manually set the battery type each time which was a nuisance. It is important to have a lower terminal charging voltage on gel batteries (14 v is pushing it) or you will destroy them. The two I bought drew battery power when unplugged from AC. This is bad bad bad. One took a nice 35A gel battery down to 4.5 volts.
There is a use case to charge 7-35A gel batteries from portable mesh cameras. These need a low charge rate (1A is good). I found a $18.99 Black & Decker “automatic” battery maintainer- BM1B. It stops at around 13.6V – good. Has two pin SAE disconnects. Good. But draws 1ma when unplugged from the battery. Bad.
So we switched to marine multi bank chargers. These are 6-25 amps. They track each battery separately. Hooking batteries together in parallel is popular but a bad idea. A common battery failure mode is a short- Battery A shorts/dies takes Battery B with it to the grave. Our chargers have just one setting for conventional/gel for all banks.
Marine chargers have been available on Craigslist locally. One had one bad ring terminal and two bad inline fuses. These can deliver up to 20A from the generator or shore power to the battery plant. Marine chargers in my experience (needs testing) do not draw battery power when unplugged from AC.
So the idea is you are are running at Field Day and have an HF radio going off the batteries all day, the shore power charger will pull from any available AC generator. At events, we may be handed a generator feed. At the 2021 Loppet Winter Festival we were set up near a booth with a 115V AC outlet. And you do not need to rely on dodgy $35 UPS units. The little $5 trickle chargers can maintain batteries when plugged in as can a solar panel. A diode on a solar panel is a good precaution.