This is pretty much the heart of our system, we have 2 agm batteries wired in parallel, and grounded through the floor of the van to the chassis. It's all wired with 2/0 stranded wiring, and I can isolate each one with the switch that controls battery 1, battery 2, or both together. Each deep cycle agm is rated for 235 amps.
This is a 750 watt inverter/ charger, with the charger rated at 20 amps. A relatively cheap one, but not the cheapest. I have learned not to fish from the bottom of the barrel, having gone through 3 inverters now. I spent about $350 on this one.
This is the other system, dedicated only for running the pulley that operates the rear ramp door. These batteries are rated for 135 amps each, wired in series, and require venting. Which means I have to keep them enclosed with a vent to keep the gases from building up in our living area when charging them.
I've also gone through a couple of 3 stage "smart chargers," but have found I do not need any more charging capacity than the cheap 1.25 amp battery tender I have permanently connected to these batteries. Of course, it's only operable when we are plugged in.
The 6 volt batteries are encased in a cheapo walmart gray storage bin, and have a "convection" vent system. One upper, and one lower vent. The cover for the bin is always on unless I am checking the health of the batteries.
This is the lower vent which provides the fresh air flow which helps the whole venting process. You can see the charging lines from the charger and also the lines to the 12 volt pulley.
Since I could not find any flexible material to use for the vent, I used PVC. It exits through the van wall with a 3 inch stub to an exterior vent. All sealed with epoxy.
OK, back to the agm batteries. This is the cheap fuse block that holds the 5 amp fuses for all the 12 volt stuff in the van. One heavy gauge line runs from the Perko switch common terminal to the fuse block.
Here you can see the Perko switch set to both agm's, the ANL fuse from the inverter to the switch, and the line from the switch to the 12 volt fuses.
Most of this system, with the exception of the 6 volt batteries, is in the open, on the floor (each agm battery weighs in at 135 lbs.), but in our box truck van we have the room. The system is not perfect, but it works for us. Since our living area is separated from the cab, we are completely "stealth" when or if we ever have to park in a city, like when I have to keep Deb at the hospital for a couple of days. But we are primarily boondockers, I like the wide open spaces. After about 7 or 8 days, the batteries are down to about 50 percent, and we go to a national forest campsite or the like and plug in overnight. It takes about 3 hours to completely charge up.
The rear ramp, which is great to open up in the heat of the summer. I used to run my cruiser motorcycle up in there before I got a trailer.
I know many of you are interested in the cheap and economical way to have power when you are off the grid, but it still has to be effective and not present any kind of fire hazard. The truck/ van we have is set up with a 30 amp input, many will not need anything more than 110 to charge a couple of batteries. Just remember to fuse everything, do your homework on capacities, and don't run cheap wire!