I always tell people who want to add batteries that it is as easy a jump starting a car. Black to black, red to red.
We often camp in cold weather, sometimes in the dead of winter. Extra battery reserve to run a furnace is often a requirement.
Here's the way I've done it on three of our campers.
This is our '05 Bigfoot 10.6e. One big deep cycle marine battery with two Hawker SBS40 compact all fits nicely in the right rear compartment.
This is a typical front mount for a travel trailer. In this case it's a Heartland MPG. It came with one small marine battery, I simply connected a pair of Hawker SBS40. The batteries are placed in simple black boating battery boxes & secured to the carrier behind the propane tanks. I do suggest you drill 1/4" drain holes in the corners. I found towing in the snow or rain, that the moisture swirls it's way into the battery box. Note also, the connections are clean. I add a coating of NO-OX-ID to them to prevent corrosion.
We just bought a "used" 2016 Forrest River Nitro 5th wheel toy box. First thing I did was to install more battery capacity. It came with one battery in the right front compartment. The new connection is on the left side with red tape over the clamp.
I was able to route the new red lead through a frame tube at the top of the compartment. To prevent abrasion of the insulation on the wire I shoved a piece of gray plastic electrical conduit (top of pic) through the square tube to the left front compartment.
In the left compartment, I drilled a hole into the steel panel and used a self tap screw to make the black (ground) connection. Red coming in from the upper right is attached to again, a pair of Hawker SBS40 compact batteries.
I got those batteries from the job I retired from. We were required to change them out when their capacity no longer met company specifications. They still have many serviceable years but back up power for electronics required they be replaced usually each 3-4 years. The pair in the above picture are dated 2007.
Originally Posted by Wankrl7
I have been researching two batteries for my new trailer.
In numerous places I have read that using two batteries of different age and condition is a poor idea. They will constantly try to equalize their voltage causing wear. I am sure somebody will set me straight though.
While it is true to an extent and was mentioned in a follow up post - equalization can be an issue for grossly different capacity batteries -
So long as the batteries you are using are "healthy" they will work together and find their own balance.
In the above pics, the Hawker batteries will usually charge to 13.74vdc. The interstate battery, by itself was 13.44vdc. When combined, all three floated at 13.63vdc when full charged.