Let's first say that heaters in cold weather can use a lot of amps. You could have batteries that are in pretty good shape but you ARE killing them if you let them get below 12.2 volts which is where recharging MUST start. 9.8 volts even under load is a dead flat battery. Batteries IF fully charged will lose about 10% of their charge a month in cold weather. More in hot weather so if they sat without charge for 11 months...they have been damaged if not made useless. They definitely need to be tested. Take them to a battery shop for testing or follow the proper steps to test them correctly yourself with a voltage meter or hydrometer.
If you have a battery charger and a voltmeter:
1. Turn the battery charger on and measure voltage AT the chargers terminals...should be minimum 13.6v to around 14.5 volts. Then check the voltage at your battery terminals...should be the same. If not ...you may have a bad connection preventing proper battery charging or a shorted battery. Pull and replace if so.
2. Assuming all is well with the charging system...leave it on ALL night to insure a full charge to your batteries.
3. In the AM...pull the negative wires off the batteries and let them sit for 24 hours.
4. After 24 hours ...measure the voltage at the battery terminals. It should be 12.6 if the batteries are healthy. Anything around 12.4V indicates significant loss of capacity. Anything 12.2 or below indicates severe damage...buy new ones.
Assuming you need new ones ...you'll need to figure out what size you have now OR if you have more room for BIGGER batteries in the space available.
1. Assuming the dealer installed 12V batteries...the most commonly used are group 24 and group 27. Possibly 31. If you see ANY of those numbers on the black beauty batteries...that is your size.
2. Alternatively...measurements are a giveaway. length, width and height
of each of these 3 sizes are:
group24... 101⁄4 x 6 3⁄4 9 7/8 typical 75 -80amp hours
group27...12 3/4 x 6 3/4 x 9 7/8 typical 90-95 amp hours
group27... 13x 6 3/4 x 9 5/8 typical 100-105 amp hours.
There will be minor variations due to case design but assuming 12V...your tape measure is your friend. NOW while you have the tape measure out...measure the space you have available for your house batteries.
IF you can accomodate: 10 1/4 x 7 1/4 x 10 7/8 pair...NOW is a chance to make the switch to a golf cart 6V pair to make up a deep cycle 220 amp hour pair @ 12V....at great prices from Sams and/Costco.
Hope this helps...good luck with the process...and since you apparently do boondock...suggest you get a REAL battery monitor so you don't kill the new ones in short order. Like this:
Victron BMV 700 Battery Monitor