That's a good idea, using the plug in volt meter. I've been scratching my head for a while now how to easily and cheaply add a meter to check my batteries. Duh! Why didn't I think of that? Already have 2 points to plug into in the trailer.
On a side note, the new charger I installed (mentioned back in this thread) to replace the pos parallax is still working great after a 2 month outing in the trailer. Yet another pos walmart deep cycle battery bit the dust. We were in the Keys and no deep cycles handy, so I just unhooked the bad battery and went back to a single battery which worked fine for the balance of the trip.
The theory on the pos walmart batteries was that we travel all over the place, and there is a walmart everywhere to warranty if we have trouble, but the problem with that theory is we have nothing but trouble with the pos walmart batteries. That makes 3 bad batteries in 2 1/2 years (dual system). Which they have warrantied so far, but just a pain in the ass. And the last one they gave me has no warranty at all, apparently they don't warranty deep cycles anymore. So I'm going to pitch both in the trash and buy 2 good batteries. Any suggestions out there on the best batteries to use? I'm set up for group 27 batteries but can modify the mounting if needed.
I had the same stupid theory on my dually, and I just had the 4th battery in 5 years go bad the first time it got cold for the winter. They would have warrantied it yet again, but I am tired of that and pitched it and bought a good one instead. No walmart batteries! Don't do it!