My battery was on the way out and it was time to get a new one. I decided to replace it with two batteries so that they will last longer when I'm boondocking.
If you're going to do multiple batteries you need to make sure you buy them all at the same time because your bank is only as good as your oldest battery. For an RV you also want to make sure you get Deep Cycle batteries. For this post I did two 12 volt deep cycle marine batteries.
Before leaving I took a picture of my old battery. For some reason positive is green and negative is white so I wanted to make sure I had a picture in case I forgot.
Then I measured the area that my trailer tongue had for the batteries. It was 25" in the narrowest place so I knew that was the maximum width the batteries in the boxes could be.
Make sure you bring your old battery to the store with you when you buy the new ones. Most places have a "core fee" (mine was $18 per battery) that gets waved if you bring back an old battery. Since I was returning one battery and buying two, I had an $18 core fee for the second battery.
I was hoping to get two group 27 batteries but they ended up being too wide so I had to go with group 24.
The two group 24 batteries fit like a glove.
I wanted to wire them in parallel so that the group was still 12v but had twice the capacity. To do this you need to wire the positive to positive and negative to negative on the two batteries. To verify you did it correctly, use a multimeter and touch the red end to the red on one battery and the black end to black on the other battery and you should get about 12v.
I used two 4 gauge wires that were 18" long and already had the terminal connections on the ends. If I were to do it again I would get them longer. Having them this short makes it kind of hard to get the lids on the battery boxes.
Now that they are wired together and you have verified you're getting the correct voltage, you just need to screw on the camper wire. Screw the positive wire onto the positive post of one battery and the negative wire to the negative post of the other.
That's really all there is to it.
Now when you hook up alligator clips to the batteries (like for a solar panel) you want to hook one clamp on each battery.
And now your battery should last twice as long!
Update 8/16/19 - Cable Management
After wiring all this everything worked great, but I couldn't get the lids on the battery boxes. I ended up drilling some holes in the boxes to run the wires and now the lids close and everything looks much cleaner.