What I did in my 5th wheel when I added an additional 12v battery was pretty simple. I have the same battery boxes you have in your pic. I bought a a 2" PVC "T" connector and 2- 2" 90 degree PVC elbows and basically ended up with a 2" "T" with a 90 degree elbow on each end. I slipped the end of the elbow over the vent flange on each battery box and attached the original vent tube to the middle flange on the "T". The cost for the PVC "T" and elbows was about 5-6 bucks at Home Depot.
Just measure the distance between the two battery box vents and buy a your "T" accordingly. If I remember correctly, the one I bought was 6 inches end to end. If you want I can take a pic of my setup for and email it to you.
The 1-12 volt is going to be iffy with what you want to do. It might make it, it might not. 2-6volts would be a big improvement. Trojans have a great rep but it comes with a cost which may be overhyped. You could buy 2 GC 6 volts (x brand) at Sam's or Costco for around $150-160 and get much longer service. Good luck!
Comments