12v battery system

Depends on how you camp and your energy consumption. Personally, I would not have a trailer with 1 battery as it's so easy to go to 2 and have the extra margin of power. Kind of a no-brainer IMO

If it's large enough to have brakes as collie said, you need it for the breakaway brake apply switch. Do the axles have brakes? Is there a steel cable lanyard at the tongue which you connect to the tow vehicle?

To meet the minimum you would need a group 24 battery which is about the smallest basic size. From there you would go up in size to a grp 27 then a grp 29 and 31. After that it's common for some of us to use two 6 volt batteries in series to create a large 12 volt battery. The gc2 6 volt is also considered a deep cycle and has a thicker plate design than a grp battery so you can run it down more and get more cycles out of it. I'd probably throw a grp 27 on it. It will meet the basics yet also give you a little more capacity if you were to do an overnight of dry camping. If you dont need it immediately keep reading up on this stuff for a while. Is there a chance that you may like dry camping and getting away from the crowds? Is there a chance you'd like to not hook up as much and pay campground fees? If so you may want to go with two sixes or you could add another grp 27 i parallel.

Either way, you then need to consider how you will recharge. If you are in a campground all the time it's pretty much not an issue. If you like dry camping you may be able to stay out and have enough capacity from your chosen battery and recharge when you get home and plug in for awhile before heading out again.

If you extend your dry camping and run a generator you then get into needing to possibly up grade your converter or install some solar or even portable solar. You may not fully charge while doing this but it could let you stay out for your desired time frame and completely charge up when you get home. It's not one size fits all.

You should check the model of your converter as some older ones are hard on battery life. If you want to run a resi frig I'd be looking at at least a few hundred watts of panels on the roof. Depending on roof space even more. Panels and pwm controllers are cheap. You pretty much need just as many holes in the roof to install an 80 or 100 watt panel as you do for a 160 watt panel.

So I just Googled and a random energy star rated 4 cubic foot fridge was rated at 630Wh per day which is 53Ah at 12V which is about 1/2 of a pair of GC2 6V batteries, basically no fridge if you're dry camping unless you want to go dual batteries AND generator, or for a lot less cost put in a 2 way fridge =)

You have to have at least one battery for the breakaway switch when towing. Makes a big difference if you plan on a inverter to run mini frig. Just lights and water pump, no furnace you could go a lot longer. Lots of variables so can't give a definitive answer. A inverter running a frig could only be a few hours. Just running led lights might last a week.

OK thanks for the replies I just bought this camper a few months ago. Kinda would like the battery just for back up. Also the fridge is 110 only

Are you certain about the fridge? Back in that day it seems to me many were 3 ways...120, 12 and propane. But even running it on propane will use some power. To power the trailer breakaway safety feature you'll need a battery. I believe you need to study. Try Battery University.

1. 4 days. I don't know either.

Why do you feel the need to add a battery now?