In my Coleman pop up I had 1 group 29 Walmart battery 122 ah per their specs and I ran my furnace and a double overhead light with the standard 12v auto bulbs and I would drain it down to 50% capacity or less before recharging and would be lucky to get 2 years out of a battery.
In our fifth wheel it came with a single Centennial DC24MF battery rated at
Reserve Capacity 25 Amp Draw 140min
Amp Hour @ 20HR Rate 75amps
I would have to recharge daily, this battery installed by the RV dealer when I bought my trailer used was way to small a group 24 automotive size battery, but why was it to small ? Lets look at battery's for a minute and for the moment we will stick with the 12 volt ones.
First most Walmart batteries are Marine/RV like my Centennial and are not true deep cycles, they have CCA (cold cranking amps) listed on them and as in the case of the Walmart batteries their AH (amp hour) rating is wrong they use a (1Amp @ 12V) draw and the standard is a 20A draw. These batteries are typically in the 100 ah to 140 ah size which only really have 50-70 amps of usable power.
Here is a good rule to go by for using battery power, for longest battery life do not drain down past the 80% mark so on a battery like my Centennial that would be only 28 amps used before recharging. This will give you the most life out of your battery it also means daily recharging for a single batty system.
Since I was using Marine batteries I used a automotive 12 volt charger run off my generator at a 30 amp max. So I tied a Walmart group 29 battery to my Centennial in series for about 240 ah battery capacity, not the best thing to do with the battery's, but it gained me a extra day before recharging and I wasn't draining the batteries as much.
This talk of AH, CCA and draws my be confusing to you as it was to me when we first got our fifth wheel it didn't even concern me with our pop up until I had to drive over a 100 miles to get a new one so we could finish our trip. So lets talk about what these things mean and it is the same for 6 volts or 12 volts.
An amp hour (AH) is a rating usually found on deep cycle batteries. The standard rating is an amp rating taken for 20 hours. What this means for a 100 AH rated battery is this: Draw from the battery for 20 hours, and it will provide a total of 100 amp hours. That translates to about 5 amps an hour. (5 x 20 = 100).
This is how many amps you have to use and is more important than battery volts, remember going down to 50% kills your battery's stay above 80% if possible. So if you have 2 100 ah battery's you have 40 amps of usable power before recharging, we consume between 20-30 amps a day so you would need to recharge every 1 1/2 days.
Reserve Capacity (RC) is a battery rating. This is the number of minutes a fully charged battery at 80°F will discharge 25 amps until the battery drops below 10.5 volts. It may be expressed as
Minutes of Discharge @ 25 Amps
I don't see this rating as very important I mean who pulls that much power continually while camping. If you were buying battery's and looking at them side by side I would go with the higher rating between the 2 simply because it means it has more capacity ie: more power in it.
A battery cycle is one complete discharge from 100 percent down to about 50 percent and then re-charged back to 100 percent.
This will give you a estimate of the usage you will get out of the battery, so if it has 100 cycles and you discharge it to 50% every 2 days then recharge you will only get 200 days from it, keep it to 80% and get a year.
Here are sum typical batteries you will see in RV's.
Duracell Ultra SLI 27MDC
Capacity 20hr: 90AH
Interstate Batteries SRM-24 Marine/RV Deep Cycle Battery
140min Reserve Capacity (RC)@25 Amps
Interstate 27DC marine/rv
Reserve Capacity(RC): 160min
Trojan T-1275 Plus
It's ratings are.
Amp Hour @ 20HR Rate 150 amp
Capacity 25 amps 280 min
I went with 6 volt batteries knowing I was going with a 4 battery bank at the start and not Trojan.
US Battery US 2200 6 volt
474 Min @25Amps
As you can see 6 volt batteries have a lot more power than even good quality 12 volt and they are comparable in cost to the better 12 volt battery's, the down side is you have to buy 2 to get 12 volt power and you will need more wiring too.
I will follow this post with one on wiring for a battery banks.