The problem that Benz and many others have is they look at the amp draw and then look at the batteries, say a 10 amp draw and a 100 amp hour battery and it should last 10 hours. But it doesn't. The Peukert effect makes the simple math worthless. It's like MPG, the faster you suck amp hours out of a battery, the few available amp hours you have.
A 100 amp hour battery with a 10 amp draw becomes an 81.23 amp hour battery (or, the 10 amps actually become 12.31 amps, same thing). But that's draining the battery dry, which will dramatically shorten the lifespan of the battery, and that's assuming really high-end true-deep cycle batteries. If it's a truck battery, which is a hybrid (like a marine battery), the lifespan is even shorter. In either case, if you run a battery down to the point where lights dim, inverters scream, air conditioners silently stop running, because of the amount of lead sulfate that has been stripped off the lead plates of the batteries, the best you can hope for is to recharge them to perhaps 90 amp hours, and that's assuming that you are recharging them properly. Most trucks will have a 30 amp charger for batteries, so the simple math will tell you that it takes 3 hours to put back 30 amps hours into the battery, except the Peukert Effect works in reverse, too, plus there's the resistance factor as the batteries start to get near fully charged. They might be as much as 85-90% fully charged after 3 hours, but it might take another 2-4 hours to get that last little bit of amp hours back in there.
The Dometic 10,000 BTU air conditioner is 10 amps, but that's at 120 volts AC, which translates to 110 amps at 12 volts. In order to run that for 8 hours you would need 1100 amp hours of batteries, and in order to keep from running the batteries down below 50% Depth of Discharge, you'd need 2200 amp hours of batteries. That's a lot of batteries. 1100-1200 pounds worth, easy. That's why 1500 watt electrical space heaters don't work out here, either.
Most 12-volt DC air conditioners will draw somewhere between 20 and 40 amps (half load and full load), at 12 volts. Most of the time, except during really hot days in someplace like Laredo, you'd run it at full load for 30-60 minutes to cool things down, then the rest of the time at half load to keep it cool. A rule of thumb for those air conditioners is to figure a 25 amp draw over the course of 8 hours. In that case, 250 amp hours of batteries will last 8 hours, and you'd need 500 amp hours of capacity to keep the batteries above the 50% DoD, assuming those batteries have no other duties. If the batteries run lights, fridge, whatever, you'd have to add those amp hour requirements to the numbers, then double it to keep things at 50% DoD.
