Battery under load

Since you're measuring at the electric panel, the voltage drop is not just from the battery itself, but potentially also partly from wiring between it and the panel. I would suggest comparing the voltage (under load) at the load center and at the battery terminals themselves. You may well have a cruddy connection somewhere--possibly a ground connection.

It does sound as though the battery is not entirely up to snuff, though; about 30 AH and it's discharged down a fair bit more than one would expect. Still, it should get you through a mild night, I think.

Just to follow up I ran a test yesterday to see how long these batteries would hold up. Once again, relying on a $9 voltmeter connected to the main 12v lugs in the electrical compartment. However that meter matches my handheld voltmeter.
I disconnected it from 120v, let it rest for 30 min. At that point was still reading 12.8. Instead of running the furnace fan I turned on all the lights till I read 7.1 amps at the battery. That brought the voltage down to 12.3. In 45 minutes it dropped to 12.2. After an hour and 20 min. dropped to 12.1. It settled there for the next 3 hours when it dropped to 12.0.
The plan was to let it dip to 11.9 before terminating and seeing what the rebound voltage was but it was 9:30 and 6 hours in so I decided to quit, still reading 12.0. Waited 30 minutes and the voltage had rebounded to 12.3.
So I plugged it back in and went to bed. I now have some evidence of how long they will last and also what to expect in terms of voltage drop under load.
Thanks everyone for your input.

Is that 7 amp load off the rating plate/brochure or have you measured that?

Estimating battery total power by using voltage is problematic. Depending on what you have for batteries, voltages can swing hard from an anticipated 12.6V (full charge) or hold voltage for long periods at indiscriminate values. If the batteries are a bastard pair or there are wiring incongruities, voltage readings become even more skewed. A stand-alone battery monitor/amp counter is a good thing to have, much more accurate than OEM measurement panels, especially those with idiot lights.

Mandoguy wrote:
As far as using the heater it would probably only be around 45 outdoors at the coldest and would like to keep inside at least at 55. I haven't done this before so that's what I'm trying to figure out. I would guess it would run no more than 3-4 hours.
Time2, I like your style. However doing hydrometer tests are a real pain on this unit as the batteries are in a tight lower compartment.
I installed a volt meter connected to the main lugs of the elecrical panel. Cheap meter but it matches my multimeter and solar controller.

If your furnace and RV are anything at all like mine, you'll be running far less than a 50% duty cycle on the furnace to keep a ten or fifteen degree difference between outside and inside...assuming you don't leave a window wide open or something like that. If you did have a 50% duty cycle there, you would be unable to maintain 55 degrees with anything lower than about 35 degrees or so outside.

I would guess maybe a 10% to 20% duty cycle...though of course there are many, many variables.

So read that jot it down
Go outside use the mm to read the batteries directly
How much difference ?
Do it again after you have four hours of heater run time ?
Or 8 hours of the heater cycling

There is your voltage spread
Now you know what the batteries really are
Ad there spread difference to your indoor reading, to track voltage

As far as using the heater it would probably only be around 45 outdoors at the coldest and would like to keep inside at least at 55. I haven't done this before so that's what I'm trying to figure out. I would guess it would run no more than 3-4 hours.
Time2, I like your style. However doing hydrometer tests are a real pain on this unit as the batteries are in a tight lower compartment.
I installed a volt meter connected to the main lugs of the elecrical panel. Cheap meter but it matches my multimeter and solar controller.

I heard that if you want to quickly de-sulfate a flooded cell battery, drop it on the floor from table height. That breaks loose the sulfation from the plates.......:p

What is the heater cycle
If your talking continous run all night, your going to wake up cold one morning

You talk like it's coming on and running for hours on end
Your mentioning battery voltage under load at one hour mark

Technically, with new batteries 7 amps *8 hours = 56 amp hours approx 30% of capacity, leaving you with plenty of battery
But that is good batteries with no sulphation
We can't really answer your question
You still didn't answer, where are you measuring voltage

You want us to give you a specific number, not to go below, when to turn off heat and bundle up , or start charging
We can't do that

Just use them hard and use them up. I am not giving the furnace a rest because of a number on the voltmeter.

If the batteries seem low on capacity then charge them to where you think they are full and verify with a hydrometer. The 12.6 start voltage may not be indicating a full charge. If you are not periodically charging at 14.4 to 14.8 volts the batteries are likely undercharged and have lost capacity.

I still plan to run more testing with the 7 amp load. I'll let you know how it goes.
Not until next June that any of this will be put to the real test though, camping off grid.