tpi, that is a great idea about actually putting the meter on the battery itself when we have a lot of stuff running off the little inverter -- that will tell me if this is just a wiring issue or if it also involves the batteries.
I know that I can run the batteries in parallel, but I have never had a problem in 800 nights of boondocking during the last ten years. We do some pretty serious off-grid camping (although almost never for more than a week at a time, due to the need to dump the tanks and do laundry and buy groceries). Yes, hooking them up in parallel would give me more capacity, but I have not needed that capacity.
Having one simply as a spare is part of my "belt and suspenders" ultra-cautious mentality -- if the one in use dies, it won't drain the other one, which is waiting patiently in its own battery box. I swap them out after a few days (monitoring state of charge) and then recharge the depleted spare.
Also, since I got a 120 volt portable solar panel, the battery is topped off during the day, almost every day, which delays the need to swap out the spare.
