You said: You have 2 six volt batteries in series.
I saw: You have one big 12 volt battery
Simply ignore the jumper that connects the two of them DO NOT connect anything to that wire or to the terminals on the ends of it. Use the most negative (That runs off to chassis ground) and the most positive (That runs off to who knows where (your fuse block) and you are looking at a 12 volt battery.
Treat them as though they were one 12 volt battery for all purposes save removal and replacement, You can do that half a battery at a time.
Now, that 12 volt battery charge outlet on your honda.. FORGET IT. it's junk you are much better off feeding say a Progressive Dynamics 9260 with the 120 volt outlet on that honda. Faster and better charging. Or the converter in your trailer if it's a good 3-stage. (Such as the one I named) note there are MANY just as good.
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times